docs: refresh architecture and development docs

Update docs content to match current launcher/plugin/runtime structure and fix stale home demo media assets with cache-busted URLs plus poster refresh. Add supporting backlog/subagent tracking records and docs asset regression coverage.

backlog/tasks/task-112 - Fix-docs-demo-video-thumbnail-cache-staleness.md

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+---
+id: TASK-112
+title: Fix docs demo video thumbnail cache staleness
+status: Done
+assignee:
+  - codex
+created_date: '2026-02-23 03:41'
+updated_date: '2026-02-23 03:43'
+labels:
+  - bug
+  - docs
+  - media
+dependencies: []
+priority: medium
+ordinal: 112000
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Docs home page demo continues to show an old thumbnail after replacing the demo video file in place. The page currently reuses static media URLs (`/assets/minecard.webm`, `/assets/minecard.mp4`, `/assets/demo-poster.jpg`), so browser/CDN cache can keep stale poster/video preview data.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Action Steps
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Add a failing regression check that asserts cache-busted URL usage for docs home demo media references.
+2. Update docs home demo media URLs to include a shared asset version token for poster/source/fallback paths.
+3. Run the regression check and docs build validation.
+4. Record fix notes and move task status to Done.
+<!-- SECTION:PLAN:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 Docs home video references include a deterministic cache-busting token so direct asset replacement does not serve stale preview data.
+- [x] #2 Poster/source/fallback links in the demo block use the same token to avoid mixed old/new media.
+- [x] #3 Validation commands for the new regression check and docs build pass.
+<!-- AC:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+- Generated a fresh thumbnail from `docs/public/assets/minecard.webm` into `docs/public/assets/minecard-poster.jpg`.
+- Updated `docs/index.md` demo block to use a shared `demoAssetVersion` token across poster, webm/mp4 sources, and gif fallback links.
+- Follow-up: regenerated `minecard-poster.jpg` at exact `00:00:12` from the webm and bumped token to `demoAssetVersion = '20260223-2'`.
+- Added regression check `docs/index.assets.test.ts` to ensure demo media references keep shared cache-busting URL patterns.
+- Validation:
+  - `bun test docs/index.assets.test.ts`
+  - `bun run docs:build`
+<!-- SECTION:NOTES:END -->