fix(spawn): acquire crew worktrees via treehouse get --lease

[nbdSteve]

Problem

treehouse v2.0.0's interactive treehouse get opens a subshell inside the worktree ("Type 'exit' to return"). tmux's #{pane_current_path} does not report that subshell's cwd, so fm-spawn.sh's worktree-discovery poll never observed the pane leaving the project directory and timed out at 60s on every crewmate/scout spawn - even though the worktree was created successfully. This blocks all crewmate dispatch on the current treehouse release.

Fix

Acquire the worktree via treehouse get --lease as a subprocess (path printed to stdout, banners to stderr), then cd the pane into it. This is the same durable primitive fm-home-seed.sh already uses for secondmate homes, and fm-bootstrap.sh already requires --lease support - the crewmate/scout path was simply left on the old interactive approach. Spawn drops from a 60s timeout to ~2s.

Also fixes a latent collision: when firstmate runs inside a tmux session whose name is a plain integer (e.g. 58), tmux new-window -t 58 is parsed as a window index, so a second task fails with index 58 in use. Addressing the session as <session>: forces session interpretation (correct for named sessions too).

Tests

• Updated fm-spawn/backend tests to the lease contract (treehouse stub prints the worktree path on get --lease).
• Converted the spawn old-vs-new byte-identical conformance test to a forward contract test, since this is an intended behavior change.
• Added a regression test proving spawn succeeds purely from the lease, with no pane-cwd read.
• Full suite passes locally; shellcheck clean on changed scripts.

Verified end-to-end: real crewmate spawns now complete in ~2s and launch into the leased worktree.

[nbdSteve]

Follow-up commit (668a703): the post-lease cd into the worktree is now verified and retried rather than fire-and-forget. A freshly created pane can drop the first keystroke (cd <wt> → d <wt>), leaving the agent in the primary checkout; the spawn now reads the pane cwd back and re-sends until it lands. Also makes fm-spawn-batch hermetic against a live config/crew-dispatch.json and adds a dropped-cd regression test.