docs: plan a pixel-art desktop mascot ("Cluck")

Design-only (no code yet): an MBSE subsystem model for a chicken mascot
that roams the desktop shell, is draggable, opens a Sims-style nested
radial menu, and has a tamagotchi lifecycle (egg -> chick -> adult) that
reacts to real app activity (chat streaming, knowledge-graph writes,
signals). Everything (animations, autonomous behaviors, menu actions,
environment reactions) is scoped as a data-driven registry for easy
extension.

- docs/mascot/README.md: subsystem Model conforming to docs/mbse's
  Holt-based Framework — mission/boundary, requirements, structural view
  (module registry map), behavioral view (behavior FSM + lifecycle state
  machines + a stimulus sequence diagram), interfaces view (which web
  stores it observes, read-only), extension guide, verification view.
- plans/2026-07-20-desktop-mascot.md: the concrete file-by-file
  implementation plan for web/src/lib/mascot/ derived from the model,
  with an ordered build sequence and a manual browser verification
  checklist.
- Indexed both in docs/index.md and plans/index.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plans/index.md

@@ -18,6 +18,7 @@ went sideways, open an investigation.
 | 2026-07-14 | [Activity timeline](2026-07-14-activity-timeline.md) | In Progress |
 | 2026-07-17 | [Codebase review, lint audit, and documentation maintenance](2026-07-17-codebase-review-and-cleanup.md) | Report delivered — doc/tooling fixes applied; code refactors pending |
 | 2026-07-18 | [Session review: three recent sessions](2026-07-18-session-review-three-sessions.md) | Implemented in v0.7.12 — P0.1/P0.2/P1.3/P1.4/P1.5/P1.6/P1.8/P2.10; P1.7 and P2.9 deferred (retry cap covers) |
+| 2026-07-20 | [Desktop mascot ("Cluck")](2026-07-20-desktop-mascot.md) | Planned — not started |
 
 ## Done