feat: add stateless changeset merge and propagation

[shaug]

TL;DR

Adds stateless, resumable merge-and-propagate commands for carved changeset chains, with explicit mutation authority and fresh GitHub safety checks before every remote update.

Summary

• Add dry-run-by-default propagate and merge-propagate commands that select an exact PR or changeset index and rehydrate state from git and GitHub rather than local plan/state files.
• Support rebase and cherry-pick propagation, resume partial propagation from a fresh clone, and rehydrate a deleted merged branch from verified PR metadata.
• Fence every mutation with fresh PR identity, ownership, base, remote-head, and merge-lineage checks; constrain force-with-lease pushes to downstream changeset branches.
• Verify the exact merged PR and commit on remote main before propagation, update downstream PR bases and (i of N) titles by explicit PR number, and skip already-completed updates safely.
• Consolidate GitHub PR payload decoding while preserving source-branch filtering and exact-number verification.

Safety

• Remote mutation requires both --no-dry-run and --ack-merge-and-propagate.
• Base, source, and merged upstream branches are never force-pushed.
• Downstream pushes use exact leases and are immediately reauthorized against live GitHub and remote state.
• Merge targets must be open, same-repository, exact-head candidates based on mainline, with every preceding changeset already verified on mainline.

Validation and review

• Exact candidate: 925affa807c203824127a0fe5e0fb084f14f378d
• Base: 2c82f2ed6c7ca10f1797656938325df18d7e6f96
• just format: passed; candidate remained unchanged.
• just test: passed, including 84 carve-changesets tests.
• git diff --check: passed.
• Repository-owned aggregate review: schema-valid clean result with zero findings for this exact candidate and base.
• Frozen skills/prepare-changesets/: zero diff.

Authorized lint exception

just lint reaches skill validation and stops only because skills/carve-changesets/SKILL.md is intentionally absent pending the packaging child (#35):

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

This exact limitation is authorized for #33; all preceding lint checks pass. No other lint failure is waived.

Tickets

Fixes #33