docs(20): document CI persistent cache host-mount dependency

- Add "CI dependency caches" subsection to CI Pipeline Overview
- Lists /pnpm-store and /ms-playwright container paths
- Notes act_runner config.yaml container.options host-mount requirement
- Clarifies that CI still works without the mounts (ephemeral fallback)

docs/DEVELOPMENT.md

@@ -205,6 +205,21 @@ Every PR to `main` runs through `.gitea/workflows/ci.yml`. A `changes` path-filt
 
 All five jobs must pass (or be legitimately skipped) before a PR can merge. See [docs/TESTING.md](TESTING.md) for test suite details.
 
+### CI dependency caches
+
+CI uses two persistent cache paths inside job containers:
+
+| Path             | Content                                                                 |
+| ---------------- | ----------------------------------------------------------------------- |
+| `/pnpm-store`    | pnpm content-addressable store (`--store-dir /pnpm-store`)              |
+| `/ms-playwright` | Playwright browser binaries (`PLAYWRIGHT_BROWSERS_PATH=/ms-playwright`) |
+
+These paths must be bind-mounted from host directories in the act_runner `config.yaml`
+`container.options` field — that is a **host-side change, not tracked in this repo**. Without the
+mounts, CI still works correctly — pnpm creates an ephemeral store at `/pnpm-store` inside the
+container and Playwright downloads browsers fresh each run. The mounts only eliminate repeat
+downloads across runs.
+
 ## Drizzle Migration Workflow
 
 Schema changes follow a strict two-step process. **`drizzle-kit push` is not available** — it has been removed from the scripts because it emits a false destructive diff (table truncation) on populated MariaDB databases.