mr-668: fold multi-graph work into v0.6.0 (no separate v0.7.0 release)

The branch had bumped workspace versions to 0.7.0 and added a
dedicated `docs/releases/v0.7.0.md` for the multi-graph work.
Per scope decision: ship the graph-rename and the multi-graph
mode in one v0.6.0 release.

Changes:

* Workspace versions bumped 0.7.0 → 0.6.0 in every crate manifest
  (`omnigraph`, `omnigraph-compiler`, `omnigraph-policy`,
  `omnigraph-server`, `omnigraph-cli`) and their internal
  `path = ..., version = "..."` dependency constraints.
* `docs/releases/v0.7.0.md` content merged into
  `docs/releases/v0.6.0.md`, retargeted to a single coherent
  v0.6.0 release note covering both the graph terminology rename
  and the multi-graph server mode. The original v0.7.0.md is
  deleted.
* All `v0.7.0` / `0.7.0` doc and comment references throughout
  `crates/`, `docs/`, `AGENTS.md`, and `openapi.json` retargeted
  to `v0.6.0` / `0.6.0`. `Cargo.lock` regenerated to match.
* OpenAPI spec regenerated via `OMNIGRAPH_UPDATE_OPENAPI=1
  cargo test -p omnigraph-server --test openapi
  openapi_spec_is_up_to_date` — `"version": "0.6.0"` now.

Verification:

* `cargo build --workspace` — clean (6 pre-existing engine
  warnings only).
* `cargo test --workspace --locked` — zero failures across all
  39 test result groups.
* `bash scripts/check-agents-md.sh` — passes (34 links / 33 docs).
* `grep -rn "0\.7\.0\|v0\.7\.0" --include='*.rs' --include='*.md'
  --include='*.json' --include='*.toml' .` returns no workspace
  hits. The three remaining `0.7.0` strings in `Cargo.lock`
  belong to unrelated 3rd-party crates (`pem-rfc7468`, `radium`,
  `rand_xoshiro`).

The git tag and crates.io publish happen later — this commit
just consolidates the surface so the eventual release is one
coherent v0.6.0 covering all the work since v0.5.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

openapi.json

@@ -7,7 +7,7 @@
       "name": "MIT",
       "identifier": "MIT"
     },
-    "version": "0.7.0"
+    "version": "0.6.0"
   },
   "paths": {
     "/branches": {