feat(pills): add OpenCode declared pill definition and accent color

[Totopo27]

Summary

Adds OpenCode (agent_opencode) to the declared workspace pill catalog (PillCatalog.swift), documents it in docs/AGENTS.md, and registers its brand accent color (#5EEAD4) in the Windows/Linux hook relay.

Closes #163

Motivation & Root Cause

Users working with OpenCode (e.g. running DeepSeek models or autonomous multi-agent pipelines) can use the --agent opencode relay argument (coucou_agent), but OpenCode was not registered as a declared pill in PillCatalog.swift. Without a catalog entry, it could not be pinned in Settings as an active or main workspace pill alongside Cursor, Codex, and Antigravity, and defaulted to a randomized fallback color in the web UI.

Key Changes

• NotchBuddy/Sources/App/PillCatalog.swift:
  • Added .init(id: "agent_opencode", name: "OpenCode", color: "#5EEAD4", category: .workspace, subtitle: "Integration", source: .agent, githubOnly: true).
  • Added sessionSubtitle mapping for "agent_opencode" returning "OpenCode".
• NotchBuddy/Sources/App/IslandViewContent.swift:
  • Added "agent_opencode" jump target pointing to terminal apps.
  • Added placeholder coming soon status in IntegrationCardView.
• windows/src/island/hooks.ts:
  • Mapped opencode to #5EEAD4 in AGENT_COLORS so the dynamic agent pill on Windows/Linux uses OpenCode's brand accent color.
• docs/AGENTS.md:
  • Documented agent_opencode among the declared pills available in the GitHub build.

Verification

• npx tsc --noEmit clean on frontend.
• npm run build completed cleanly.