Expose pull request iteration identity in get_changes #1500

Adds the remote-documented repo_pull_request:get_changes action to the consolidated local repository tool using the smallest page-level contract needed to bind review data to an exact pull request iteration.

Each bounded page preserves { iterationId, nextSkip, nextTop, changes } and adds sourceRefCommit, targetRefCommit, and commonRefCommit, plus iteration reason, retarget names, and explicit commit/change continuation indicators. Callers can pass the returned iterationId, nextSkip, and nextTop to continue paging without changing identity.

Iteration reasons use the stable lossless shape { value, names, unrecognizedBits }. Known flag names are deterministically ordered, Push is represented explicitly for zero, Unknown appears only when its SDK bit is set, absent reasons preserve a nullable shape, and any unrecognized integer bits remain visible.

The implementation validates commit IDs and pagination metadata, caps get_changes pages at 1,000 entries without changing the existing list schema, and re-reads both the latest and selected iteration after retrieving changes. It fails closed if either identity moves. It does not use compareTo, target-tip comparisons, synthetic merges, FileDiffs, write operations, or additional OAuth scopes.