Show classifier cost in the auto-mode status line

[bsamek]

Summary

• Accumulate classifier usage cost from fast, detailed, and retry responses across the current session.
• Show the total in the AM status line after the first classifier call, and expose it through status inspection.
• Persist and restore the total with session state, default older persisted entries to zero, and clear it with /automode reset.
• Document the status-line field and add coverage for multi-call/retry aggregation, restore compatibility, formatting, and reset.

This PR contains only the cost/status-line feature; classifier reasoning changes remain in #58.

Validation

• npm run check — passed.
• Targeted status and hook tests — 12 passed.
• npm pack --dry-run — passed.
• Full npm test — 316 passed, 2 failed, 1 skipped. The two failures are the hard-deny/path-normalization assertions in tests/hard-deny.test.ts and tests/hook-path-policy.test.ts; the same failures occurred on this machine before implementation.