Detect working/waiting agents in worktrees (reuse worktrunk's activity markers) #86

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 Skip worktrees with a live process inside them #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 Skip worktrees with a live process inside them #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/Skip worktrees with a live process inside them #66
  logic still applies.
• No [worktrunk] section, or no marker for this branch → falls back to
  Skip worktrees with a live process inside them #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).