docs: add project skill and Directus extension playbook

- project skill: invariants, presets/formats, backfill/scheduler,
  debugging, verification gate, deploy checklist
- AGENTS.md points at the project and global skills

.gitignore

@@ -3,4 +3,6 @@ node_modules
 dist
 dev/extension
 dev/.smoke.env
-.opencode/
+.opencode/*
+!.opencode/skills/
+!.opencode/skills/**

.opencode/skills/directus-image-previews/SKILL.md

@@ -0,0 +1,119 @@
+---
+name: directus-image-previews
+description: Work on the directus-extension-generate-image-previews bundle (Directus 11 image transform pre-generation). Use when changing the warmer, backfill, scheduler, admin module, endpoints, presets, deployment, or when debugging preview generation in this repo.
+---
+
+# directus-image-previews — working rules
+
+Read this before changing the extension. It encodes hard-won invariants; breaking them silently
+produces cache misses or production incidents. For general Directus-extension patterns, see the global
+`directus-extensions` skill.
+
+## Scope
+
+A Directus **bundle** (tested on **11.17.4**, manifest `host: ^11.0.0`) that pre-generates image transform
+cache files:
+
+- `src/hook/` — `files.upload` warmer + in-process nightly `schedule('*/15 * * * *')`.
+- `src/endpoint/` — `/image-previews/*` admin API.
+- `src/module/` — admin page (progress, storage, config, diagnostics, API reference).
+- `src/lib/` — settings, state, presets, warmer, backfill, diagnostics, storage, window, format.
+- `dev/` — isolated Directus 11.17.4 stack + deploy/rollback scripts.
+
+## Invariants — do not regress
+
+1. **Cache filename** = `<basename(filename_disk)>__<object-hash(resolved transforms)>.<ext>`, beside the
+   original in the storage root.
+2. **Hash = `object-hash@3` defaults** (sha1/hex, `unorderedArrays:false` → array order matters,
+   `unorderedObjects:true`), over the **resolved** transform array.
+3. **`resolvePreset` order**: `transforms[]` → `toFormat` (if `format`/`quality`) → `resize` (+`extract`).
+4. **Never pass `{ key }` alone** — `resolvePreset` ignores `key`. Always pass the full preset object.
+5. **`format:'auto'` needs an explicit `acceptFormat`** (`avif`/`webp`); otherwise server-side resolves to
+   jpg/png and the hash won't match the browser.
+6. **Preset registry mirrors the controller**: `[...SYSTEM_PRESETS, ...storage_asset_presets]`, first match
+   wins. `GET /settings` returns only custom presets.
+7. **`AssetsService.getAsset` awaits the write**; destroy/unpipe the returned stream to free the FD.
+8. **Local storage writes are not atomic** — never generate the same file from two workers concurrently.
+9. **State dir** is `<STORAGE_LOCAL_ROOT>/.image-previews` (writable). Do not use
+   `/directus/extensions/...` (root-owned → EACCES).
+10. **Env values from the Directus context are coerced** (booleans/numbers). Stringify before parsing.
+11. **All authoritative state is file-backed** (PM2 cluster). Never in-memory.
+12. **Hooks never throw into Directus core**; all behavior is gated by `enabled`.
+
+## Presets / formats semantics
+
+- System presets are all `format:'auto'` → one file per configured `formats` (default `avif,webp`).
+- Custom preset with a fixed `format` → exactly that format, `formats` is ignored.
+- Custom preset with `auto` → follows `formats`.
+- Custom preset with no `format`/`quality` → resize-only (original extension, still hashed).
+- A custom key equal to a system key is **shadowed** (system wins) and flagged by `/presets`.
+
+## Backfill / scheduler semantics
+
+- `Run now` → `POST /run` with `ignoreWindow:true`; runs until cancelled.
+- Nightly → `schedule('*/15 * * * *')`: only inside the maintenance window, no active run, and not already
+  completed today. It starts/resumes the backfill.
+- The backfill **pauses** when the window closes (`run.paused=true`) and resumes next night.
+- Pre-scans storage filenames and counts existing preview files as `already cached` (skips generation).
+- `total = image files × tasksPerFile`; `done` counts cached + generated + failed.
+- One run at a time (in-process guard + `run.lock`).
+
+## State files (on the Directus host/container)
+
+```
+<STORAGE_LOCAL_ROOT>/.image-previews/
+  settings.json   effective config
+  run.json        progress (running/done/total/generated/hits/failed/rate/eta/paused/cancelled/error)
+  state.jsonl     completed {file_id,preset_key,format} → resume
+  run.lock        cluster-wide run lock
+  cancel.flag     cancellation request
+```
+
+## Debugging
+
+- `GET /image-previews/status` — progress, window, paused, last error.
+- `GET /image-previews/presets` — effective formats, shadowed keys.
+- `GET /image-previews/resolve-plan?fileId=&key=` — predicted cache filename, no write. **Use this first.**
+- `POST /image-previews/generate` `{fileId,keys?,formats?,dryRun?}` — one file, per-task results.
+- `GET /image-previews/ops?limit=` — recent operations with errors (e.g. unsupported preset format).
+- Directus log (file transport): `Added extensions`, `Extensions reloaded`, `image-previews: …`.
+- Common failure: custom preset format not supported by sharp (e.g. `bmp`) → error in `/ops`; fix the preset.
+
+## Verification gate
+
+Before shipping a change that touches generation, prove:
+
+1. `resolve-plan` filename for `?key=system-medium-cover` is byte-identical to the file produced by a real
+   browser request with `Accept: image/avif`.
+2. `npm test` green, `npm run typecheck` clean, `npm run build` emits both bundles.
+3. Isolated `dev/` stack: upload → 12 files (6 presets × 2 formats), backfill idempotent, pause/cancel works.
+
+## Build / deploy
+
+- **Never build on the production host** (no Node; npm is unreliable there).
+- Build locally: `npm ci && npm run build && npm test`.
+- Package `dist/` + `package.json` (+ README/LICENSE) into a tarball; SCP it.
+- Install: copy `dist` **first**, `package.json` **last**; back up the previous `dist`.
+- `EXTENSIONS_AUTO_RELOAD=true` in prod — never copy a half-built extension into the watched dir. If a
+  reload looks ambiguous, restart the container.
+- Rollback: restore the backed-up `dist` or remove the extension directory.
+- Public repo: `git@code-at-locus.im:Rawman/directus-extension-generate-image-previews.git`, branch `master`.
+  `.opencode/` internal notes are untracked; only `.opencode/skills/` is tracked.
+
+## Checklist for any change
+
+- [ ] Invariants above untouched (or explicitly justified).
+- [ ] Tests added/updated; 43+ green; typecheck clean.
+- [ ] `resolve-plan` hash-match still proven.
+- [ ] No new in-memory state; state dir still writable.
+- [ ] Endpoint stays admin-guarded; no secrets/file contents logged.
+- [ ] Deploy payload is `dist` + `package.json`; backup taken; reload verified.
+- [ ] README/AGENTS updated if behavior or settings changed.
+
+## Links
+
+- Global skill: `directus-extensions` (bundles, context APIs, storage, transforms, deployment).
+- Docs: https://directus.com/docs/guides/extensions/overview
+- Transforms: https://directus.com/docs/guides/files/transform
+- Source anchors: `api/src/services/assets.ts`, `api/src/utils/transformations.ts`,
+  `api/src/controllers/assets.ts`, `api/src/constants.ts`.

AGENTS.md

@@ -10,6 +10,14 @@ admin UI never pays the first-request sharp cost.
 Bundle entries: an api **hook** (`files.upload`), an api **endpoint** (`/image-previews/*`), and an app
 **module** (`image-previews` admin page: progress, storage, config, diagnostics, API reference).
 
+## Skills
+
+- **Project skill**: `.opencode/skills/directus-image-previews/SKILL.md` — invariants, presets/formats
+  semantics, backfill/scheduler behavior, debugging, verification gate, deploy checklist. Read it before
+  changing generation logic.
+- **Global skill**: `directus-extensions` — reusable Directus extension playbook (bundles, context APIs,
+  storage, asset transforms, scheduling, sandboxing, deployment, links). Use it for new extensions.
+
 Tested against Directus **11.17.4** (self-hosted, PM2 cluster, local disk storage). The manifest targets
 `^11.0.0`; the extension uses non-public `@directus/api` internals at runtime, so re-verify on each
 Directus minor upgrade.