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>

docs/index.md

@@ -8,6 +8,7 @@ procedures, and the system model.
 | ---- | -------- |
 | [adr/](adr/README.md) | Architecture Decision Records (numbered, append-only) |
 | [mbse/](mbse/README.md) | Model-Based Systems Engineering views of the platform |
+| [mascot/](mascot/README.md) | MBSE subsystem model for the desktop mascot (planned) |
 | [operations/](operations/README.md) | Operator runbooks (deploy, rollback, recovery) |
 
 For agent orientation see [AGENTS.md](../AGENTS.md); for the operating model