feat: first-class OpenCode integration via a local plugin

[arzafran]

What this does

Completes #139: OpenCode gets the same first-class treatment as Claude Code and Codex. OpenCode sessions now register with Programa, show Running and "Needs input" status in the sidebar, send completion and attention notifications, and clean up properly when the session ends. Installed the same way as the others: "Install OpenCode Integration…" in the File menu (or programa opencode install-integration), which shows exactly what will be written and asks first.

Closes #139.

Summary

• OpenCode has no shell-hook config — its extension surface is a JS plugin API. The integration is a small local plugin (embedded in the CLI, ~30 lines) written to ~/.config/opencode/plugins/programa.js (respects OPENCODE_CONFIG_DIR). Local plugin files auto-load: no opencode.json edit, no npm install — simpler than the existing omo machinery. Design validated against opencode.ai docs and sst/opencode source; event mapping: plugin boot → session-start, chat.message → prompt-submit, session.idle → stop, permission.asked → needs-input, dispose → session-end.
• Every plugin shell-out is .quiet().nothrow() — a programa CLI failure can never crash the user's OpenCode session.
• programa opencode-hook mirrors the codex handler: opencode/opencode.<sessionId> PID keys, statuses, notifications, guaranteed session-end cleanup. It deliberately mirrors codex-hook's quiet-on-missing-socket guard; verification showed claude-hook lacks that guard and exits 1, so codex's is the pattern that matches the plugin's fire-and-forget contract.
• Uninstall has one extra safety rule: it refuses to delete the plugin file if the managed-by marker comment is missing — a file the user replaced with their own content is never destroyed.
• Menubar item + en/ja localization, mirroring the Claude/Codex entries.
• Known quirk documented in the handler: OpenCode's session.idle and permission.asked can fire close together; status application is idempotent like the other agents' handlers, so no dedupe machinery was added.

Test Plan

• tests_v2/test_opencode_install_integration.py 6/6 against the built CLI, no app required (fresh install, idempotent reinstall, modified-file diff + restore, uninstall delete, marker-refusal with file intact, decline-untouched)
• Debug app build succeeds
• Live end-to-end with a real OpenCode session (needs opencode installed; the hook handler's socket calls are the same exercised paths as codex-hook)
• CI green