feat: harvest crew-generated untracked files into the project on teardown

[jonathan-edgar]

What

On a ship teardown, fm-teardown now copies every untracked, non-ignored file the crew generated in its worktree into the project's primary checkout (same relative path, never overwriting an existing file), then removes it from the worktree.

Why

treehouse return hard-resets the worktree, so crew-generated notes/docs/scratch were lost on teardown - and their presence made teardown refuse as "dirty." This preserves them and lets teardown close out cleanly.

How

• harvest_untracked_into_project() runs before the safety check, on ship teardowns only (scout worktrees are scratch; secondmate homes aren't project worktrees).
• Uses git ls-files --others --exclude-standard, so firstmate's own gitignored hook files are never harvested.
• Purely additive at the destination, no git-state change. A file that can't be copied is left in place, so a copy failure surfaces as the normal dirty refusal rather than a silent loss.
• Committed-but-unlanded work is untouched and still refuses teardown (prime directive fix(spawn): require the resolved task worktree to belong to the project #3 intact).
• AGENTS.md: recorded as the seventh sanctioned write exception (section 1) with a teardown note (section 7).

Validation

• shellcheck bin/*.sh bin/backends/*.sh tests/*.sh clean.
• tests/fm-teardown.test.sh passes, incl. 3 new cases: copy-into-project (allows an otherwise-dirty teardown), no-clobber preserves existing, scout exemption.

Note: raised directly (no-mistakes not installed locally); the 'Require no-mistakes' check will flag.