Hardening: artifact cleanup policy and final output schema validation

[qinhaihong-red]

Parent

#1

What to build

Hardening for v0.1 release: a configurable artifact cleanup policy with a safe default,
and schema validation of final workflow outputs at report time. Closes the Phase 6 exit
criterion: a real repository workflow produces a reviewable report with trace evidence.

Acceptance criteria

• A configurable artifact cleanup policy (such as keep-last-N or age-based) with a conservative default that never touches the active run
• Final workflow outputs are validated against a declared schema and violations are reported clearly
• A real repository workflow produces a reviewable report with trace evidence

Blocked by

• Reporters and caw report in four formats #12

[qinhaihong-red]

Folded in: adapter-side artifact capture (from #11 / PR #101 review)

A review of #101 (codex.exec) flagged that agent Adapters don't capture artifacts.
Confirmed symmetric, not codex-specific: neither the merged claude.print (#9) nor
codex.exec (#11) populates AgentResult.artifacts — both return the default empty tuple.
The Adapter interface docstring already attributes "full artifact lifecycle" here, so making
that ownership explicit as the capture front-half of this issue's artifact work:

• Real Adapters (claude.print, codex.exec) capture the files a writable agent run
  produces and populate AgentResult.artifacts so they are indexed in State — alongside
  the cleanup policy + final-output validation already listed.

Single authoritative owner for artifacts; #11 stays symmetric with #9 and does not regress capture.