Delete managed worktree branches on cleanup

[plx]

Human comments

Like #2725, this was discovered by Fable while I had it onboarding me to the codebase and assessing the route to some adjustments/plugins I was curious about making for myself.

Since this bug also seemed to check out, I had Codex verify and then open this PR, too.

What was wrong

removeWorktree removed managed checkout metadata and the directory but never deleted the branch created by git worktree add -B. Successful cleanup therefore left every generated bb/<slug>-<threadId> branch in the source repository, contrary to the worktree guide's cleanup contract reported in #2724.

What changed

• Read the worktree's symbolic HEAD immediately before removal while holding the existing checkout and worktree-metadata locks.
• After a successful git worktree remove, delete that branch against the resolved common Git directory while also coordinating through the shared Git-ref mutation lock.
• Skip branch deletion for detached HEAD or failed symbolic-ref lookup, and never delete the branch when worktree removal itself fails.
• Report branch deletion failures and Git diagnostics in the environment destroy transcript without failing worktree destruction.
• Cover successful cleanup and same-name reprovision, detached HEAD, a branch checked out by another worktree, failed setup rollback, and failed worktree removal.
• Document the non-fatal branch-deletion failure behavior.

The server↔daemon command and result schemas did not change, so HOST_DAEMON_PROTOCOL_VERSION is unchanged.

How you verified

• Fail-before: the new successful-destroy/reprovision test, failed-setup rollback assertion, and deletion-failure transcript test failed against the old implementation; detached-HEAD and failed-worktree-removal guard tests passed as safety invariants. All pass with this change.
• TMPDIR=/private/tmp pnpm exec turbo run test --filter=@bb/host-workspace --force --env-mode=loose — 9 files, 233 tests passed. Loose env mode forwards the normalized macOS temp path around the existing /tmp vs /private/tmp assertion on main.
• pnpm exec turbo run typecheck --filter=@bb/host-workspace --force — passed.
• pnpm exec turbo run test --filter=@bb/server --force -- --run test/environments/environment-provisioning.test.ts — 11 tests passed, including preservation of the stored branch name during managed reprovision.
• Live source QA on origin/main: after managed environment destruction, the worktree disappeared but git branch --list still returned bb/branch-cleanup-repro-thr_wzberxex69.
• Live source QA with this branch: after the same spawn/archive/destroy flow, both the worktree and bb/branch-cleanup-fixed-thr_x4z2wru6gz were gone.
• pnpm exec oxfmt --check docs/worktrees.md packages/host-workspace/src/provisioning.ts packages/host-workspace/test/provisioning.test.ts
• git diff origin/main...HEAD --check

Fixes #2724

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