docs: define carve-changesets suite handoffs

[shaug]

TL;DR

Defines how each carved changeset is reviewed and handed to the existing PR babysitter without duplicating lifecycle or propagation ownership.

Summary

• Define the raw, candidate-bound review packet for each stacked changeset, including its exact predecessor base, goal, non-goals, specifications, and prefix/full validation evidence.
• State when repository-owned review is required, when it is not applicable, and how head changes or base drift invalidate evidence.
• Map publish authority to ready_to_merge with merge withheld and merge-and-propagate authority to merge_when_ready with explicit merge authority passed through.
• Define the ownership boundary in both directions: the babysitter owns delegated CI and feedback lifecycle work, while carve-changesets retains decomposition, rehydration, renumbering, and propagation.
• Map all babysitter terminal results back to carve-changesets progress or fail-closed states, including the rehydrate-propagate-next-PR sequence after a verified merge.
• Add carve-changesets and both review paths to the repository dependency overview.

Non-goals preserved

• No Python or scripted handoff-packet generation.
• No changes to babysit-pr or review-code-change.
• No lifecycle implementation duplicated from the delegated skills.

Validation and review

• Exact candidate: 2df5136e2a7226666bc136e30905c2442a579c78
• Exact base: 0bf347b887350397a7d73047d16eb3bc8ec100fe
• just format: passed; candidate remained unchanged.
• just lint-py: passed.
• just lint-md: passed.
• git diff --check: passed.
• just test: passed, including 84 carve-changesets tests.
• Repository-owned review suite: solution simplicity, correctness, code simplicity, and aggregate results are schema-valid clean with zero findings for the exact candidate and base; the reviewer left the candidate and worktree unchanged.

Authorized lint exception

just lint stops only at the temporary pre-packaging condition owned by #35:

Validation failed for skills/carve-changesets:
  - Missing required file: SKILL.md

This exact limitation is authorized for #34. All preceding lint checks pass; no other failure is waived.

Tickets

Fixes #34