feat: add relay cost attribution estimator

[orban]

Summary

• add component invocation telemetry for relay hooks (post_tool_use, pack, pre_compact, reload, nudge)
• add configurable per-component cost model (config.cost_model) and backward-compatible state backfill
• compute/store deterministic per-component and total estimated cost in metrics.last_cost_report during pack
• track async extraction attempts/runs and attribute extraction as its own component
• document /relay costs command behavior and estimator limits in docs
• add tests for schema defaults/backfill, counter helpers, deterministic attribution math, config fallback, and report formatting

Assumptions

• this is an internal operational estimator for relay subsystem activity, not provider billing truth
• costs are attributed by invocation counts multiplied by configurable unit rates

Example output

/relay costs reports a component table: invocation count, unit cost, estimated subtotal, plus total estimated cost and assumptions.

Validation

• python -m pytest hooks/scripts/test_hooks.py -q (162 passed)

Note

Values are modeled estimates, not measured cloud billing data.