Add opencode as a declarable agent pill

[naylonmedya]

Related to #9 (opencode support request). Third-party agents already get an automatic transient pill via \coucou_agent; this makes opencode pinnable in Settings so the pill survives after the session ends instead of being removed 5 s after Stop.

Changes

• \NotchBuddy/Sources/App/PillCatalog.swift: \�gent_opencode\ entry under Agents (no \githubOnly\ — the bridge is an external plugin spawning the relay directly, so it works in every build with no hook installer) + \sessionSubtitle\ case. No status-hint branch needed: unlike gemini/agy/codex there is nothing to install, so the row shows no warning.
• \windows/src/core/state.ts: same catalog id/name/color;
  emoveTask\ now mirrors macOS \AppState.removeTask\ — a pill that is both in-catalog and checked in Settings resets to idle instead of being removed. Behavior for undeclared pills is unchanged.
• \windows/src/settings/main.ts: new Agents section listing catalog pills with \source == .agent, reusing the existing switch + 4-pill-budget pattern (shared with Integrations).
• \docs/AGENTS.md: one-line note documenting the opencode declared pill.

Verification (Windows)

• \ sc --noEmit: clean; full \ auri build\ (vite + rust + nsis): pass.
• Headless state test (18 assertions, all pass): declared+active pill survives Stop as idle with steps/badge cleared; undeclared and unchecked pills are still removed; sort order and opt-in defaults unchanged.
• Live relay test against the built app: SessionStart/UserPromptSubmit/PreToolUse/Stop arrive in \coucou.log, pill appears, and — visually confirmed on the island — stays idle after the 5.2 s window instead of disappearing.

Caveats

• The macOS Swift changes follow existing patterns exactly but could not be compiled here (no Xcode on this machine) — please rely on CI for that side.
• No screenshot/GIF available from this environment (headless Windows); happy to add one if CI artifacts allow.
• No new network, IPC, auth, secrets, or settings.json writes; \�alidateAgent\ (including the \claude\ reservation) is untouched.