feat: adopt existing cmux workspaces for supervision

[ss251]

Intent

Add an fm-adopt command so firstmate can supervise an already-running cmux workspace (kind=adopted) without spawning a new worktree or branch; peek/send/watcher work on it and teardown un-registers it without ever closing the workspace. Implements issue #260. cmux-only, experimental, supervise-only.

What Changed

• Added bin/fm-adopt.sh and tests for registering an existing cmux workspace as kind=adopted so firstmate can supervise it without spawning a new worktree, branch, or process.
• Wired adopted cmux targets into session-start, crew-state, watcher, backend, and teardown paths, including duplicate-window protection and liveness checks that do not depend on readable screen output.
• Documented the adopted-session workflow and constraints across the operator docs, script reference, architecture/configuration notes, and cmux backend guide.

Risk Assessment

✅ Low: Captain, the change is scoped to cmux adoption paths and the shared watch/peek/send/teardown integrations look consistent with the intended supervise-only behavior.

Testing

• ⏭️ Test - skipped

Pipeline

Updates from git push no-mistakes

✅ **intent** - passed

✅ No issues found.

✅ **Rebase** - passed

✅ No issues found.

🔧 **Review** - 2 issues found → auto-fixed ✅

• ⚠️ bin/fm-adopt.sh:79 - fm-adopt only rejects an existing task id, but it never rejects an existing window=<workspace>:<surface> already registered under another id. The watcher and window_to_task resolve a window to the first matching meta file, so double-adopting a surface or adopting a firstmate-spawned cmux task can make stale handling, peek/send ownership, and teardown bookkeeping attach to the wrong task or skip stale wakes entirely. After resolving T, scan state/*.meta for the same window= and refuse it.
• ⚠️ bin/fm-adopt.sh:98 - Adoption accepts a live cmux surface based on list-panes, but fm-crew-state.sh later treats non-tmux liveness as fm_backend_capture, which for cmux calls read-screen. The cmux adapter documents that read-screen fails on fresh never-written surfaces, so an adopted live workspace can immediately report backend target gone. Use fm_backend_target_exists for the adopted cmux liveness probe before falling through to busy/log classification.

🔧 Fix: Harden adopted cmux supervision
✅ Re-checked - no issues remain.

⏭️ **Test** - skipped

Step was skipped.

✅ **Document** - passed

✅ No issues found.

⏭️ **Lint** - skipped

Step was skipped.

✅ **Push** - passed

✅ No issues found.