fix(spawn): skip treehouse get's transient cwd in worktree-discovery poll

[psacc]

Problem

herdr-backend spawns intermittently aborted with:

error: treehouse get did not yield an isolated worktree (resolved '/'; ...); refusing to launch

and — worse — left a stuck treehouse get holding the pool's only slot with no crewmate attached, so the next spawn also failed (the single-slot wedge). tmux-backend spawns were unaffected.

Root cause

treehouse get transits intermediate cwds while it sets the worktree up — it is momentarily at / and at the treehouse root (~/.treehouse) before it creates and enters the pooled worktree. bin/fm-spawn.sh's worktree-discovery poll accepted the first cwd that merely differed from the project directory, so it captured / (or ~/.treehouse), handed that transient to validate_spawn_worktree, and aborted — while the still-initializing treehouse get completed and held the slot forever.

herdr's foreground_cwd exposes those transients because it reads the actively-running foreground process's cwd; tmux's pane_current_path reports the settled interactive-shell cwd and never surfaced them — which is why only the herdr path wedged.

Fix

The poll now gates on a new non-fatal is_isolated_worktree <path> predicate (a real git-worktree root, physically distinct from the primary checkout — the same rule validate_spawn_worktree now delegates to), so it skips / and ~/.treehouse and keeps polling until the pane settles in the actual worktree. No-op for tmux, whose first observed non-project cwd already is that worktree.

Testing

• Added tests/fm-backend.test.sh:test_spawn_worktree_poll_skips_treehouse_get_transient_cwd, which drives the real bin/fm-spawn.sh against a fake pane whose polls return the project dir, then a transient /, then the real worktree. It reproduces the exact resolved '/' failure against the pre-fix poll and passes against the fix.
• fm-backend and fm-tangle-guard suites green; shellcheck bin/*.sh bin/backends/*.sh tests/*.sh clean.
• Reproduced live 3× against real herdr 0.7.3 + treehouse on a single-slot pool before the fix.

Opened as a draft for maintainer review before publishing.

[psacc]

Superseded by #648, which carries the same worktree-poll fix raised through the no-mistakes pipeline (the gate this PR could not pass). Closing in its favor.