Detect working/waiting agents in worktrees (reuse worktrunk's activity markers)

Depends on #66.

Problem

#66 adds a --check-busy guard that answers one question: is any live
process's cwd inside this worktree? That's necessarily coarse — a process-table
scan can't tell a shell parked at an idle prompt from a coding agent mid-turn,
which is why #66 already has to special-case interactive shells (bash, zsh,
...) to avoid false positives, and still can't distinguish "an agent is
actively generating, uninterruptible" from "an agent finished its turn and is
sitting at a prompt waiting for the next instruction, session still open."
Both look identical from /proc/*/cwd: one live process, agent binary, cwd ==
worktree path.

That distinction matters for git sync: work truly in flight should never be
removed, but a worktree where an agent is idle-but-not-exited is a judgment
call the user could reasonably make differently from "someone's shell happens
to be parked here."

Worktrunk already solves exactly this, without a
process scan: its Claude Code / Codex / OpenCode / Gemini CLI plugins call
wt config state marker set "🤖" on session start/resume and
wt config state marker set "💬" when the agent finishes a turn and waits for
input, clearing it (wt config state marker clear) on session end (see
Activity tracking). The
marker is plain git config — worktrunk.state.<branch>.marker, value
{"marker":"🤖","set_at":<epoch>} — readable without invoking wt at all, and
it degrades predictably: a killed session just leaves a stale marker until
wt config state marker clear runs.

Proposal

Add a WorktreeGuard::Working (next to Locked / TooYoung,
src/cleaner.rs:992-1027) driven by this marker, layered in front of — not
instead of — the #66 busy scan:

1. Read worktrunk.state.<branch>.marker via the existing Git::config_get
   (src/git.rs:965). No dependency on the wt binary or
   worktrunk_available() (src/git.rs:209) — only on the git-config
   convention, so this works even for repos that never installed wt, as long
   as some agent plugin (or a manual hook, per worktrunk's own docs) writes the
   marker.
2. 🤖 (working) → WorktreeGuard::Working, skipped unconditionally, same as
   Locked — not overridden by --force, consistent with "work in flight is
   never removed."
3. 💬 (waiting/idle) → reported distinctly (new ItemStatus::Waiting,
   src/report.rs:45-62) but not a hard guard: still subject to #66's own
   busy check and the normal merged/dirty logic. This is the judgment-call
   tier — surfaced so the user sees "an agent session is open here, idle"
   instead of silence, but it doesn't block -y.
4. No marker present (repo has no [worktrunk] section, or none set for this
   branch) → fall back entirely to #66's process-table scan. The two checks
   are complementary: the marker is precise but only present where a plugin is
   wired up; the process scan is universal but coarse.
5. Same flag/config shape as --worktrunk/--check-busy: --check-agents /
   --no-check-agents, sync.checkagents config, enabled by default,
   degrading to a no-op (not an error) when the [worktrunk] section is
   simply absent.
6. Document the marker format git-wipe reads (README config table) so the
   convention is explicit rather than implicit "we happen to read worktrunk's
   format."

Scoped to reading the marker only — git-wipe doesn't run agent sessions, so
it has nothing to set.

Acceptance

• worktrunk.state.<branch>.marker = 🤖 → worktree skipped unconditionally;
  --force does not override.
• Marker = 💬 → reported as waiting, not guarded; normal merged/dirty/#66
  logic still applies.
• No [worktrunk] section, or no marker for this branch → falls back to
  #66 behaviour at the cost of one extra git config --get per worktree.
• --no-check-agents / sync.checkagents = false skips marker inspection
  entirely.
• A stale marker (agent process killed before its session-end hook ran)
  does not crash or hang detection.
• Unit tests seeding worktrunk.state.<branch>.marker via git config, JSON
  report coverage, README (flags table + config table).

Start with WorktreeGuard in src/cleaner.rs:992-1027, Git::config_get in src/git.rs:965, and ItemStatus in src/report.rs:45-62; review the existing --worktrunk and --check-busy paths first. Add marker handling, matching flags and config, unit tests for git-config markers and JSON output, and README entries. Done means working markers always skip, waiting markers are reported without guarding, absent or stale markers fall back safely, and the disable options avoid inspection.