Add backlog audit and task update command

- agents/prompts/backlog_audit.md: QA analyst prompt for checking
  which pending tasks are already implemented in the codebase
- agents/runner.py: run_audit() — project-level agent that reads
  all pending tasks, inspects code, returns classification
- cli/main.py: kin audit <project_id> — runs audit, offers to mark
  done tasks; kin task update <id> --status --priority
- web/api.py: POST /api/projects/{id}/audit (runs audit inline),
  POST /api/projects/{id}/audit/apply (batch mark as done)
- Frontend: "Audit backlog" button on ProjectView with results
  modal showing already_done/still_pending/unclear categories

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_cli.py

@@ -205,3 +205,38 @@ def test_cost_with_data(runner):
     assert r.exit_code == 0
     assert "p1" in r.output
     assert "$0.1000" in r.output
+
+
+# ===========================================================================
+# task update
+# ===========================================================================
+
+def test_task_update_status(runner):
+    invoke(runner, ["project", "add", "p1", "P1", "/p1"])
+    invoke(runner, ["task", "add", "p1", "Fix bug"])
+    r = invoke(runner, ["task", "update", "P1-001", "--status", "done"])
+    assert r.exit_code == 0
+    assert "done" in r.output
+
+    r = invoke(runner, ["task", "show", "P1-001"])
+    assert "done" in r.output
+
+
+def test_task_update_priority(runner):
+    invoke(runner, ["project", "add", "p1", "P1", "/p1"])
+    invoke(runner, ["task", "add", "p1", "Fix bug"])
+    r = invoke(runner, ["task", "update", "P1-001", "--priority", "1"])
+    assert r.exit_code == 0
+    assert "priority=1" in r.output
+
+
+def test_task_update_not_found(runner):
+    r = invoke(runner, ["task", "update", "NOPE", "--status", "done"])
+    assert r.exit_code != 0
+
+
+def test_task_update_no_fields(runner):
+    invoke(runner, ["project", "add", "p1", "P1", "/p1"])
+    invoke(runner, ["task", "add", "p1", "Fix bug"])
+    r = invoke(runner, ["task", "update", "P1-001"])
+    assert r.exit_code != 0