Wire CostTracker into project runs; persist cost_log.json for all attempts

[nandanadileep]

Closes #20

Summary

• Add get_project_cost_tracker(pipeline_dir, project_id, config=None) factory to lib/checkpoint.py
• The factory creates a CostTracker bound to <pipeline_dir>/<project_id>/cost_log.json, loaded from project config
• Existing entries are reloaded automatically across agent turns / pipeline stages
• Failed paid attempts are tracked via reconcile(entry_id, actual_usd, success=False) — they land in cost_log.json with status: "failed" so spend is never undercounted
• asset_manifest.json remains the asset inventory artifact; cost_log.json is the authoritative spend ledger

What changed

lib/checkpoint.py — added get_project_cost_tracker() factory (no changes to existing functions)

tests/contracts/test_phase0_contracts.py — added TestProjectCostTracker with 5 tests:

• Correct cost_log_path set to project directory
• Cost log file is created on first operation
• Failed paid attempts are written with status: "failed" and counted in spent budget
• Entries reload correctly across stages (cross-turn persistence)
• Different projects have isolated cost logs

Test plan

• python3 -m pytest tests/contracts/test_phase0_contracts.py -v — all new tests pass, no regressions
• Pre-existing failures are all ModuleNotFoundError: No module named 'numpy' (unrelated to this change)