feat(jobs): require schedule activation review

[owainlewis]

Summary

• keep direct Markdown job authoring while requiring a separate durable review before new or changed enabled schedules can activate
• bind approval to one persisted allowlisted channel identity and the exact validated file, execution, and delivery revision
• add fail-closed migration, restart recovery, queued-claim revalidation, durable lifecycle history, and crash-repairable audit outbox
• expose schedule review history through human and JSON push job reviews output and document the activation boundary

Why

Agent-authored Markdown should remain easy to create and inspect, but a file change must not silently create or alter unattended recurring execution.

Test plan

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo build --locked
• cargo test --locked (398 unit, 2 docs, 14 init CLI, 14 JSON CLI, 3 crash tests)
• uv run --with-requirements requirements-docs.txt mkdocs build --strict
• git diff --check
• two independent final subagent reviews with no actionable findings

Risks

• SQLite schema v12 migration captures an exact one-time baseline for existing enabled schedules
• scheduler planning and queued claims now fail closed when activation no longer matches the exact revision
• audit delivery is at least once after a crash and uses stable event UUIDs for deduplication

Related issue

Closes #169