feat(plugin): add opencode AI plugin with session-id tracking

[artyomsv]

Summary

Adds opencode as the second production AI pane type alongside claude-code. Tracks opencode's session-id rotation (new session, /new, fork, compaction) so panes resume to the same conversation across daemon restarts — the central value-prop of Quil for AI workflows.

The trick: opencode has a real first-class plugin runtime (Bun-based, 25+ lifecycle events) that we hook via the OPENCODE_CONFIG_CONTENT env var. This means no writes into ~/.config/opencode/ — the plugin file lives entirely under $QUIL_HOME/opencodehook/. OPENCODE_CONFIG_CONTENT merges with the user's existing opencode config (verified
against opencode 1.14.x), so their plugins, agents, and modes remain active inside Quil-spawned opencode panes.

How it works

1. Daemon start writes the embedded JS plugin to $QUIL_HOME/opencodehook/quil-session-tracker.js via opencodehook.EnsureScripts (atomic temp+rename, mirrors claudehook).
2. Pane spawn injects three env vars on the PTY:
   • QUIL_PANE_ID=<paneID>
   • QUIL_HOME=<absolute-path> (absolutized at the boundary so a relative QUIL_HOME cannot silently break tracking)
   • OPENCODE_CONFIG_CONTENT='{"plugin":["<abs-path-to-quil-session-tracker.js>"]}'
3. Plugin hooks session.created / session.updated / session.idle / session.compacted / session.deleted events from opencode's bus, extracts event.properties.sessionID (with fallback to event.properties.info.id for forward-compatibility with the SDK type defs), and atomically writes $QUIL_HOME/sessions/opencode-<paneID>.id.
4. Daemon restore reads that file via opencodehook.ReadPersistedSessionID and promotes the resume args to ["--session", "{session_id}"]; falls back to the plugin's configured ["--continue"] if no id was recorded yet.

Files

Area	Files
New package	internal/opencodehook/ (opencodehook.go, opencodehook_test.go, scripts/quil-session-tracker.js, o_nofollow_unix.go, o_nofollow_windows.go)
New plugin TOML	internal/plugin/defaults/opencode.toml
Daemon wiring	internal/daemon/daemon.go (EnsureScripts call, opencodeSpawnPrep, opencodeResumeTemplate, refactored resumeTemplateFor into per-plugin dispatch, templateHasPlaceholder helper)
Tests	internal/daemon/spawn_args_test.go (6 new cases) + 29 cases in opencodehook_test.go
Docs	.claude/CLAUDE.md, docs/plugin-reference.md, CHANGELOG.md
Backport	internal/claudehook/claudehook.go (one comment improving cleanup intent)
Build artifacts	.gitignore (added /quil-dev, /quild-dev, /quil-debug, /quild-debug)

Hardening applied during review

• TOCTOU closed — ReadPersistedSessionID uses O_NOFOLLOW (build-tagged constant for portability) instead of Lstat-then-Open
• Shape validation on read — opencodehook.IsValidSessionID (Go mirror of the JS plugin's regex) filters corrupted ids before they reach --session <arg>
• Absolute-path enforcement — BuildConfigContent rejects relative paths so a non-absolute QUIL_HOME cannot silently break tracking when prompts_cwd puts opencode in a user-chosen directory
• JS log rotation — $QUIL_HOME/opencodehook/hook.log capped at 1 MB with single rotation; writes de-duplicated by last recorded id so session.updated bursts during a single response don't thrash the disk
• JS plugin defensive coding — typeof sessionId === "string" guard, quilHome.includes("\0") rejection
• Pane-id regex alignment — Go paneIDRe and JS PANE_ID_RE aligned so a future pane-id format change cannot silently disable tracking via JS-only rejection
• templateHasPlaceholder — fixes a real bug where session_scrape with empty PluginState dropped its --continue fallback on restore
• NUL byte in ValidateQuilDir's reject set — would otherwise truncate env vars silently on the C side
• atomicWrite Chmod-before-Write — closes the umask-derived Windows window where the temp file briefly has 0644

Test plan

• go vet ./... — clean
• go test ./... (full suite, 17 packages) — all green
• go test -race on internal/opencodehook/, internal/daemon/, internal/claudehook/ — clean
• GOOS=windows go build ./... — clean (validates o_nofollow_unix.go / o_nofollow_windows.go build-tag split)
• node --check internal/opencodehook/scripts/quil-session-tracker.js — JS syntax valid
• Manual end-to-end smoke (macOS arm64):
  • Daemon write of plugin file to $QUIL_HOME/opencodehook/
  • Opencode picks up plugin via OPENCODE_CONFIG_CONTENT (verified via opencode debug config showing source: OPENCODE_CONFIG_CONTENT)
  • Session-id recorded on first reply
  • Daemon restart respawns pane with --session <id> arg (verified in spawn log)
  • After deleting the recorded id, restart falls back to --continue (verified in spawn log)
  • --print-logs toggle preserved across restart

Caveats / Future work

• No SQLite probe for stale session ids — opencode handles stale ids itself with a clear error; we may revisit if it proves too noisy. SQLite path documented in code: ~/.local/share/opencode/opencode.db.
• Idle handler patterns in opencode.toml are copied from claude-code; should be tuned after dogfooding against opencode's actual prompts.
• --pure toggle deliberately not exposed — would disable external plugins including our tracker.