fix(hooks): crate_guard.py rejects Cargo.toml writes inside .claude/worktrees/ (blocks /clud-pr workflow) #794

Sub-issue of #789, prerequisite for Phase 1 (#790).

Symptom

/clud-pr (and any worktree-based workflow under .claude/worktrees/)
cannot edit Cargo.toml files. crate_guard.py denies the write with:

Refusing to create/modify Cargo.toml at
'.claude/worktrees/<branch>/crates/fbuild-build': fbuild is kept close
to a monocrate ...

even though crates/fbuild-build IS in the allowlist.

Root cause

ci/hooks/crate_guard.py:relative_dir resolves the edit path relative
to os.getcwd(), which the harness pins to \$CLAUDE_PROJECT_DIR (the
main checkout, not the worktree). So an absolute path like
<repo>/.claude/worktrees/feat-xyz/crates/fbuild-build/Cargo.toml
yields rel_dir = ".claude/worktrees/feat-xyz/crates/fbuild-build",
which is not in APPROVED_CRATE_DIRS and gets denied.

Fix

After computing rel_dir, strip a leading .claude/worktrees/<single path component>/ prefix if present, then re-check against
APPROVED_CRATE_DIRS. A worktree under that path is by definition a
fbuild checkout, so the same allowlist applies.

Equivalent regex:

_WORKTREE_PREFIX_RE = re.compile(r"^\.claude/worktrees/[^/]+/?")
posix = _WORKTREE_PREFIX_RE.sub("", posix, count=1)

Acceptance

• crate_guard.py accepts edits of allowlisted Cargo.toml paths
  inside .claude/worktrees/<any-branch>/crates/<allowed>/.
• Still rejects new top-level crates (e.g.
  crates/fbuild-new-thing/Cargo.toml from either main or
  worktree).
• New unit test in ci/hooks/ (or wherever the hook tests live)
  exercises both the worktree-allowed and worktree-blocked cases.