docs(releases): finalize v0.7.0 notes (#259)

Reconcile the v0.7.0 release notes with what 0.7.0 actually ships. The draft was
mid-cycle; two facts changed and a late-cycle arc was missing.

- omnigraph.yaml is REMOVED (not deprecated): drop the deprecation-window
  framing (config migrate, OMNIGRAPH_NO_LEGACY_CONFIG, OMNIGRAPH_SUPPRESS_YAML_
  DEPRECATION); the two-surface config (cluster.yaml + ~/.omnigraph/config.yaml)
  is the only config.
- Cluster-only server: the server boots only from --cluster; no single-graph
  flat-route / positional-URI / omnigraph.yaml-graphs boot. Deprecated-route Link
  headers are the sibling-relative form (<load>, not </load>).
- Add the RFC-011 tail: defaults.store, profile list/show, schema-apply refusal
  (CLI signpost + server 409), read-only aliases, the any/served/direct/control/
  local capability vocabulary, removed legacy data-plane addressing.
- New "Engine & substrate" section: Lance 6→7, indexed traversal, scalar-index/
  query-latency, index-materialization-deferred, recovery liveness, branch-fork
  self-heal, composite @unique.
- New "Embeddings (RFC-012)" section + breaking bullet: provider-independent
  client (OpenRouter default), @embed same-space validation, the default-provider
  flip (OMNIGRAPH_EMBED_PROVIDER=gemini for Gemini-direct; OMNIGRAPH_GEMINI_BASE_URL
  dropped).
- Upgrade notes: replace the false "omnigraph.yaml keeps working / config migrate"
  guidance with the manual cluster.yaml + operator-config path; add server
  --cluster and embeddings notes.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: aaltshuler