Add git replay and batch-CAS primitives to the forge git plane #46

Outcome

Give the forge the git primitives a stack service composes: ancestry checks, merge-base reads, tree merges, commit replay, and atomic multi-ref updates. Nothing here knows what a pull request is.

Context

docs/stacked-prs.md sections 12.1–12.4, 13.7, and 7.4 define the target surface. Today OpenAgents.Forge.Repos.set_refs!/2 runs plain git update-ref with no expected-old-value check, and read-only plumbing lives in OpenAgents.Forge.Browse. The WAL index generation (OpenAgents.Forge.WAL.cas_index/3) is the existing compare-and-swap token.

Acceptance criteria

• Read helpers resolve refs, test ancestry, and compute merge bases against the bare repo cache after a freshness check.
• Tree merges plan with git merge-tree --write-tree; conflicts return structured path data without touching refs.
• Commit replay implements the git rebase --onto boundary semantics: replay only commits reachable from the old head but not from the stored boundary.
• A batch update applies every ref or none, rejecting any mismatched expected OID. The WAL records one entry for the batch so cache convergence and mirrors see one transition.
• Boundary commits stay reachable through hidden internal refs or an equivalent retention rule that the garbage collector honors.
• All git invocations stay argv-only against --git-dir, per the existing discipline in Repos.git/3.

Verification

Unit tests per primitive plus a property-style test that concurrent writers cannot interleave a batch. Run mix precommit.

Dependencies

None. This is forge-plane work and lands before any stack schema.