feat(MR-656): inline query strings in CLI and HTTP server

CLI:
- Add -e / --query-string <STRING> to omnigraph read and omnigraph change
- Exactly one of --query, --query-string, --alias is required (3-way XOR)
- Empty --query-string is rejected with a clear error

HTTP:
- New POST /query (read-only, clean field names: query/name/params/branch/snapshot)
- Mutations on /query are rejected with 400 -- use POST /change instead
- ChangeRequest fields polished: query (alias query_source), name (alias query_name)
- POST /read and POST /change remain byte-compatible for existing clients

Tests:
- cli.rs: -e happy-path on read/change, mutex error vs --query, empty -e rejected
- system_local.rs: inline -e read and -e change exercise the local flow
- system_remote.rs: inline -e read/change over HTTP plus direct /query 200/400
- server.rs: /query 200, /query 400 on mutation, /change legacy field alias
- openapi.rs: new /query path, QueryRequest schema, ChangeRequest field-name polish

Docs: cli.md (-e examples), cli-reference.md (read/change rows), server.md (/query)
Co-Authored-By: Ragnor Comerford <ragnor.comerford@gmail.com>

docs/user/cli-reference.md

@@ -11,8 +11,8 @@ A reference for the `omnigraph` binary's command surface and `omnigraph.yaml` sc
 | `init` | `--schema <pg>` → initialize a repo (also scaffolds `omnigraph.yaml` if missing) |
 | `load` | bulk load a branch (`--mode overwrite\|append\|merge`) |
 | `ingest` | branch-creating transactional load (`--from <base>`) |
-| `read` | run named query (params via `--params`, `--params-file`, or alias args) |
-| `change` | run mutation query |
+| `read` | run named query; source via `--query <path>`, `-e`/`--query-string <GQ>`, or `--alias <name>` (exactly one) |
+| `change` | run mutation query; same `--query` / `-e` / `--alias` mutual-exclusion as `read` |
 | `snapshot` | print current snapshot (per-table version + row count) |
 | `export` | dump to JSONL on stdout (`--type T`, `--table K` filters) |
 | `branch create \| list \| delete \| merge` | branching ops |