fix(bin): make tmux target_exists answer from exact membership, not the active window

[harmeet-1337]

Summary

Fixes audit finding F-S02-1 (P0, reproduced defect): fm_backend_target_exists reported ABSENT tmux windows and sessions as present. Its tmux display-message -p -t <target> probe answers from the client's active window whenever any tmux server is running, so every consumer that treats the result as authoritative (session-start fleet digest, fm-busy-lib, fm-fleet-snapshot, the away-mode daemon's %N supervisor-target validation, fm-control, fm-send) could read a dead endpoint as alive.

Change

• fm_backend_target_exists is now a pure per-backend dispatcher, matching its fm_backend_composer_state / fm_backend_agent_state siblings, so no presence answer is derived inline.
• tmux: new fm_backend_tmux_target_exists using exact-match tmux list-panes. A session:window endpoint is anchored =sess:=win so a prefix cannot match (firstmate:fm-task never matches fm-task-2); bare %N/@N pane/window ids are queried directly. Never starts a server.
• herdr: the former inline arm, moved verbatim into fm_backend_herdr_target_exists (direct pane read, no server-ensure side effect).
• zellij/cmux: thin aliases over their existing target_ready; orca: a one-line capture probe.
• fm_backend_tmux_agent_state's three-way missing/unreadable/present membership check is deliberately left untouched.
• No call-site changes.
• Remote-secondmate digest guard: because remote secondmate records use window=remote:<id> with no backend= key (defaulting to tmux), the corrected exact-match probe would otherwise flip their session-start digest line from falsely "alive" to falsely "dead". The fleet digest now special-cases window=remote:* and prints a distinct non-locally-probed line instead of running a local existence probe, since a remote:<id> window can never match a local backend's shape.

Tests

• Real-tmux smoke test gains absent-window, absent-session, absent-%N, and prefix-collision cases.
• Session-start and the shared daemon/watcher fakes model list-panes at parity with the old presence read; the fm-send-strict tmux stub is updated likewise.
• bin/fm-lint.sh is clean (ShellCheck 0.11.0 + actionlint 1.7.12, both pinned).

Delivery note

The account has read-only access to the upstream repository, so the branch is delivered from the fork harmeet-1337/firstmate. Every commit on the branch is in scope — the original fix plus the pipeline's own in-scope review/test/document gate-fix commits (the approved remote-secondmate guard, the fm-send-strict stub update, and documentation/comment corrections). No out-of-scope tooling commit was injected, so none was excluded.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge, with no concrete changed-code failure identified.

The new dispatcher preserves prior non-tmux backend behavior, while tmux now checks exact target membership and the remote digest guard follows the repository’s established remote-endpoint contract.

_{Reviews (1): Last reviewed commit: "no-mistakes(document): document exact-me..." | Re-trigger Greptile}