Add usage and cost attribution

[imran-siddique]

What changed

• add a validated usage_record() constructor and explicit CostObservation
• add idempotent model-call accumulation with per-agent and per-workflow rollups
• label token and cost coverage, preserve cost provenance, and reject mixed currencies
• add schema constraints, negative conformance fixtures, documentation, and package exports

Why

AGT and other runtimes expose token counts, estimates, provider costs, and pre-aggregated counters in different forms. Adopters need a backend-neutral boundary that can emit per-agent and per-workflow usage without fabricating unknown cost or silently double counting rollups.

Impact

Callers can report leaf usage facts and build conservative run-level totals. Unknown pricing remains absent rather than zero; partial observations remain visible through coverage counts.

Validation

• python tools/check_versions.py
• python tools/check_schemas.py
• python -m unittest discover -s tests -v (85 tests, 20 subtests)
• python conformance/runner/validate.py
• python -m compileall -q src
• python -m pytest -q (84 tests at the preceding proof point; final unittest suite contains the added safety case)
• python -m build
• isolated wheel install and import smoke test