docs: rename runs.md/runs.rs → writes and repoint all references (#131)

The Run state machine was removed in MR-771 (v0.4.0); `docs/dev/runs.md`
and `crates/omnigraph/tests/runs.rs` have since documented and tested the
direct-publish write path, so the "runs" name was misleading.

- git mv docs/dev/runs.md → docs/dev/writes.md (reframe H1 + intro;
  keep MR-771 history note)
- git mv crates/omnigraph/tests/runs.rs → tests/writes.rs (reframe header)
- repoint every runs.md / runs.rs reference across docs, AGENTS.md, and
  source comments
- fix four pre-existing broken `docs/runs.md` links (the file never lived
  at that path) to `docs/dev/writes.md`
- fix the stale v0.4.0 anchor to the live section

No behavior change: every source edit is a comment. Engine builds and the
renamed test passes 25/25; scripts/check-agents-md.sh passes.

The run-removal cleanup itself (run_registry.rs guard, __run__ prefix) is
deferred to MR-770.

crates/omnigraph-cli/tests/system_local.rs

@@ -991,7 +991,7 @@ query vector_search($q: String) {
 
 // The publisher CAS conflict shape is verified end-to-end at the engine
 // level in
-// `crates/omnigraph/tests/runs.rs::concurrent_writers_one_succeeds_one_gets_expected_version_mismatch`
+// `crates/omnigraph/tests/writes.rs::concurrent_writers_one_succeeds_one_gets_expected_version_mismatch`
 // and at the HTTP boundary in
 // `crates/omnigraph-server/tests/server.rs::change_conflict_returns_manifest_conflict_409`.
 // A CLI-level race would be timing-dependent; with direct-publish the