sdk: migrate to omnigraph-server v0.6.0

- Bump serverVersion pin 0.4.2 -> 0.6.0; resync spec/openapi.json from
  upstream main (includes today's PR #110 + PR #119 multi-graph).
- Expose og.graphs.list() on flat GET /graphs; add GraphsResource.
- Multi-graph routing via OmnigraphOptions.graphId and og.graph(id):
  Transport rewrites graph-scoped paths to /graphs/{id}/..., leaves
  /healthz and /graphs flat.
- Add MethodNotAllowedError mapped from HTTP 405 / code:method_not_allowed.
- SchemaApplyInput auto-gains allowDataLoss (Camelize over the spec).
- PR #110 follow-up: add og.query() (POST /query) and og.mutate()
  (POST /mutate); mark og.read()/og.change() @deprecated. ChangeInput
  field rename (querySource->query, queryName->name) is intentional and
  breaking at the TS layer; wire still works via serde aliases.
- MCP package: expose graphId, register graphs_list tool +
  omnigraph://graphs resource, add canonical query/mutate tools,
  retitle legacy read/change tools, read OMNIGRAPH_GRAPH_ID env.
- READMEs: multi-graph clusters section, v0.6 auth states
  (Open / DefaultDeny / PolicyEnabled), allowDataLoss, query/mutate
  migration callout, MethodNotAllowedError.
- E2E workflow: single-graph job writes a Cedar policy and starts via
  --target/--config so non-read actions are authorized in v0.6.
  New e2e-multigraph job initializes alpha+beta graphs and runs the
  multi-graph SDK suite (gated by OMNIGRAPH_E2E_MULTIGRAPH=1).
- Tests: 81 unit + 19 single-graph e2e + 4 multi-graph e2e — all green
  against a locally-built omnigraph-server v0.6.0. check-coverage now
  binds 17 SDK call sites to 17 spec operations.