refactor(artifacts): make child outputs exact and retire stale lifecycle state

[Astro-Han]

Summary

• return only child-produced Artifact sources from child result artifactIds
• replace tombstones and status transitions with physical deletion and Session-scoped authority
• retire unused sources, global reads/deletes, change notifications, readiness shims, and duplicate metadata abstractions
• simplify SQLite Artifact metadata and discard unsupported or malformed Artifact rows without blocking Session startup
• remove Artifact-specific purge and publication recovery journals; uncommitted Artifact residue is disposable and cannot block the Runtime Host

Compatibility boundary

Artifact data from older Nightly builds is best-effort and may be discarded during upgrade. Session metadata, messages, RuntimeEvent history, and continued conversation remain outside this disposal boundary. Sources required for continuation—session effects, archived tool results, projected images, uploads, deep research output, and subagent writeback—remain durable.

Validation

• npm run build
• npm run typecheck
• npm run format:check
• npm run lint
• ASF license header audit
• complete @maka/storage suite: 1111 tests, 1103 passed, 8 platform skips, 0 failures
• Runtime Host reopen-image, archived-tool-result, Session-effect idempotency, and subagent-patch continuation tests
• source-by-source ablation tests; every retained source has a failing removal counterexample

Closes #4788.