feat(metrics): per-agent token/cost accounting

Feature 4. claude is now launched with --output-format json; the run-log trailing
result JSON is parsed (defensively, never fatal) for usage + total_cost_usd. New
idempotent ALTERs add input_tokens/output_tokens/cache_read_tokens/cost_usd to
agent_runs; the launcher monitor records usage per run, posts a per-agent finish
comment under that agent bot (e.g. Developer gotov · 45.2k in / 12.1k out · $0.21),
and the deployer posts an end-of-task summary (SUM over agent_runs GROUP BY agent)
on done. New src/usage.py holds parse/format/record/summary helpers; test_usage.py
covers parsing a real CLI JSON blob, NULL-on-garbage, recording, formatting, and the
per-task aggregate.

src/db.py

@@ -77,6 +77,13 @@ def init_db():
         "CREATE UNIQUE INDEX IF NOT EXISTS idx_events_delivery "
         "ON events(delivery_id) WHERE delivery_id IS NOT NULL"
     )
+    # Feature 4 (token usage): per-run token / cost accounting. Parsed from the
+    # claude --output-format json result by the launcher monitor. Idempotent
+    # ALTERs (no-op once the columns exist) so this is safe on the live prod DB.
+    _ensure_column(conn, "agent_runs", "input_tokens", "INTEGER")
+    _ensure_column(conn, "agent_runs", "output_tokens", "INTEGER")
+    _ensure_column(conn, "agent_runs", "cache_read_tokens", "INTEGER")
+    _ensure_column(conn, "agent_runs", "cost_usd", "REAL")
     conn.commit()
     conn.close()