Plugin System Phase 1: Runtime Slot (#9)

[tu11aa]

Summary

Phase 1 of issue #9 (Plugin/Extension System). Abstracts cmux behind a RuntimeDriver interface mirroring the existing src/drivers/ agent driver pattern, and exposes runtime ops via cockpit runtime <op> CLI so bash scripts no longer hardcode the cmux binary.

Closes the largest coupling point identified in the plugin system brainstorm (cmux-threaded through every script). Next phases (workspace/tracker/notifier slots, external plugin loading) remain deferred per spec §Non-Goals.

Spec: docs/specs/2026-04-20-plugin-system-runtime-design.md
Plan: docs/specs/2026-04-20-plugin-system-runtime-plan.md

What changed

New:

• src/runtimes/types.ts — RuntimeDriver interface + types (WorkspaceRef, RuntimeSpawnOptions, RuntimeProbeResult)
• src/runtimes/cmux.ts — createCmuxDriver() — cmux-backed impl extracted from scattered call-sites
• src/runtimes/registry.ts — RuntimeRegistry with forProject / global / probeAll
• src/runtimes/index.ts + tests (12 cmux tests, 6 registry tests)
• src/commands/runtime.ts — cockpit runtime status|send|send-key|list|read-screen|stop

Migrated to driver:

• src/commands/launch.ts — uses RuntimeRegistry (retains 3 bounded execSync calls for select-workspace/current-workspace; phase-1 exception documented in spec §Non-Goals)
• src/commands/shutdown.ts — full rewrite via registry
• src/commands/doctor.ts — now probes configured runtimes instead of hardcoded cmux check; supports per-project overrides
• scripts/execute-reaction.sh — all cmux calls → cockpit runtime send/status; 0 \$CMUX refs remain
• scripts/spawn-workspace.sh — existence-check block migrated; spawn block intentionally deferred

Config (backward compatible):

• runtime?: string at top level (global default, absent = cmux)
• runtime?: string per-project (overrides top-level)

Doc updates:

• README.md — commands table + config JSON example + Runtime Abstraction architecture note

Contract highlights

• RuntimeDriver.send() delivers AND commits (Enter). CmuxDriver pairs cmux send with cmux send-key Enter internally. Bash callers that previously did both explicitly now call cockpit runtime send once.
• status() returns WorkspaceRef | null — replaces the fragile grep-list-workspaces-and-check-empty-string pattern.

Test plan

• npm run test -- --run — 56 pass (2 pre-existing config.test.ts failures unrelated to this PR — emoji assertion mismatch that predates this work)
• npm run lint exits 0
• npm run build exits 0
• cockpit doctor shows new "Runtime 'cmux' installed" check
• cockpit runtime --help shows 6 subcommands
• cockpit runtime list returns live workspaces
• Manual smoke: cockpit launch + "New day" force-fresh path (tested by reviewer)
• Manual smoke: reactor auto-fix-ci dispatch via migrated execute-reaction.sh

Pre-PR code review

Internal review flagged one landmine — cockpit runtime send --command foo bar would silently bind foo as target and discard bar. Fixed in commit d1bdda2 with an explicit guard. Other reviewer notes (regex fragility in parseList, shell-injection surface in escape, partial-send atomicity, buildRegistry duplication) are tracked as phase-2 follow-ups per spec scope.

Follow-ups (explicitly out of scope)

• Workspace / tracker / notifier slots (phase 2)
• External plugin loading from node_modules (phase 3)
• tmux / Docker / SSH runtime drivers (each its own PR)
• select-workspace / current-workspace abstraction
• Full bash→TS rewrite of spawn-workspace.sh