feat: persist structured subagent usage

[leset0ng]

Summary

• capture each finalized assistant message as { timestamp, provider, model, usage }
• expose usageRecords on subagents:completed and subagents:failed
• persist the same records in the parent session's subagents:record entry
• keep the existing billed-token/UI semantics unchanged (input + output + cacheWrite)
• emit only newly generated records for resumed invocations to avoid duplicate analytics entries

This gives cross-session analytics extensions a durable source for subagent model attribution, cache usage, and itemized cost without reading temporary sidechain transcripts or duplicating message content.

Closes #137.

Validation

• npm run lint
• npm run typecheck
• npm run test (677 passed, 4 live tests skipped)
• npm run build
• npm run test:e2e (37 passed, 4 live tests skipped)