refactor: split agents/merge.rs into mod.rs + squash.rs + conflicts.rs
The 1772-line merge.rs is split into:
- conflicts.rs: try_resolve_conflicts + resolve_simple_conflicts + tests (351 lines)
- squash.rs: run_squash_merge orchestrator + cleanup + run_merge_quality_gates + tests (1306 lines)
- mod.rs: doc, types (MergeJobStatus, MergeJob, MergeReport, SquashMergeResult), re-exports (52 lines)
Tests stay co-located.
No behaviour change. All 20 merge tests pass; full suite green
(2635 tests with --test-threads=1).
dave
ce94dd0af4