Reconcile missing cost stack behavior

[iam-brain]

Summary

• restore missing GPT-5 pricing aliases and pro cached-read handling from the earlier stack
• recover Codex model attribution from session metadata and preserve nil-cost semantics when unknown usage is mixed in
• backfill the cost chart to a rolling 30-day window and keep partially priced or unknown-only days visible in hover details

Validation

• swift test --filter 'CostUsagePricingTests|CostUsageScannerTests|CostUsageScannerMergedStackTests|CostHistoryChartMenuViewTests'
• swift test --filter 'CodexOAuthTests|MenuDescriptorSparkTests|MenuCardModelSparkTests|StatusMenuTests|CLICostTests|CostUsageDecodingTests|CostUsageScannerCodexRecoveryTests|CLISnapshotTests'
• pnpm check
• ./Scripts/compile_and_run.sh

Notes

• this is the non-destructive reconciliation branch for the missing functionality from the earlier PR stack
• attempted subagent audit twice, but the review agent tool timed out; changes were reviewed directly before push