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.

docs/user/query-language.md

@@ -70,7 +70,7 @@ A single mutation query must be **either insert/update-only or delete-only**. Mi
 
 > `mutation '<name>' on the same query mixes inserts/updates and deletes; split into separate mutations: (1) inserts and updates, then (2) deletes. This restriction lifts when Lance exposes a two-phase delete API (tracked: MR-793 / Lance-upstream).`
 
-Reason: under the staged-write rewire (MR-794), inserts and updates accumulate in memory and commit at end-of-query, while deletes still inline-commit (Lance 4.0.0 has no public two-phase delete). Mixing creates ordering hazards (same-row insert→delete becomes a no-op because the staged insert isn't visible to delete; cascading deletes of just-inserted edges break referential integrity by silent design). Until Lance exposes `DeleteJob::execute_uncommitted`, the parse-time rejection keeps both paths atomic and correct. See [docs/dev/runs.md](../dev/runs.md) and [docs/dev/invariants.md](../dev/invariants.md).
+Reason: under the staged-write rewire (MR-794), inserts and updates accumulate in memory and commit at end-of-query, while deletes still inline-commit (Lance 4.0.0 has no public two-phase delete). Mixing creates ordering hazards (same-row insert→delete becomes a no-op because the staged insert isn't visible to delete; cascading deletes of just-inserted edges break referential integrity by silent design). Until Lance exposes `DeleteJob::execute_uncommitted`, the parse-time rejection keeps both paths atomic and correct. See [docs/dev/writes.md](../dev/writes.md) and [docs/dev/invariants.md](../dev/invariants.md).
 
 ## IR (Intermediate Representation)