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.

AGENTS.md

@@ -81,7 +81,7 @@ Full diagram and concurrency model: [docs/dev/architecture.md](docs/dev/architec
 | Embeddings (compiler + engine clients, env vars, `@embed`) | [docs/user/embeddings.md](docs/user/embeddings.md) |
 | Branches, commit graph, snapshots, system branches | [docs/user/branches-commits.md](docs/user/branches-commits.md) |
 | Transactions and atomicity (per-query atomic; branches as multi-query transactions) | [docs/user/transactions.md](docs/user/transactions.md) |
-| Direct-publish writes (the former Run state machine, now demoted to publisher CAS) | [docs/dev/runs.md](docs/dev/runs.md) |
+| Direct-publish write path (staging, D2, recovery sidecars; the former Run state machine) | [docs/dev/writes.md](docs/dev/writes.md) |
 | Three-way merge and conflict kinds | [docs/dev/merge.md](docs/dev/merge.md) |
 | Diff / change feed (`diff_between`, `diff_commits`) | [docs/user/changes.md](docs/user/changes.md) |
 | Query execution, mutation execution, bulk loader, `load` vs `ingest` | [docs/dev/execution.md](docs/dev/execution.md) |
@@ -176,7 +176,7 @@ cargo run -p omnigraph-server -- <uri> --bind 0.0.0.0:8080   # run the server fr
 
 # Run one crate / one test file / one test fn
 cargo test -p omnigraph-engine --test traversal           # one integration-test file (see docs/dev/testing.md)
-cargo test -p omnigraph-engine --test runs concurrent     # one test fn by name substring
+cargo test -p omnigraph-engine --test writes concurrent   # one test fn by name substring
 cargo test -p omnigraph-engine some_inline_test -- --nocapture   # show stdout
 
 # Feature-gated suites (each is its own job in CI, not part of the default run)