schema-lint chassis v0: code-tagged diagnostics (MR-694) (#87)

First slice of the schema-lint chassis. Adds stable `OG-XXX-NNN`
codes to schema-migration rejections so operators can suppress, look
up, and filter on identifiers rather than free-text prose. Atlas-style
chassis adapted to omnigraph's typed-IR substrate (no SQL injection
vector, no per-engine locks, native edge/vector/embedding types).

What's in v0:

- New `omnigraph-compiler/src/lint/` module with:
  - `diagnostic.rs` — Family / SafetyTier / Severity enums covering ten
    families (DS, MF, CD, BC, NM, OW, NL, VE, ED, LK). Only DS and MF
    are populated in this PR.
  - `codes.rs` — 8 DiagnosticCode constants (OG-DS-101..105,
    OG-MF-103, OG-MF-104, OG-MF-106). Five of the eight are wired to
    real emission sites; the other three are reserved.
  - Unit tests for catalog invariants: codes unique, prefix matches
    family, suffixes are 3-digit, destructive defaults to error,
    lookup() works, EMITTED_IN_V0 codes exist in ALL_CODES.

- `SchemaMigrationStep::UnsupportedChange` gains an optional
  `code: Option<String>` field. New `unsupported_error_message()`
  helper prefixes the message with `[code]` when present.

- 5 of 17 existing rejection paths now carry codes:
  - `removing node type` → OG-DS-102
  - `removing edge type` → OG-DS-103
  - `removing property` → OG-DS-104
  - `adding required property without backfill` → OG-MF-103
  - `changing property type` → OG-MF-106
  Remaining 12 paths carry `code: None` and are tagged as future work.

- `schema_apply` surfaces the formatted error (with `[code]` prefix);
  CLI `omnigraph schema plan` renders the code on the
  `unsupported change on <entity>` line.

- PR #62 destructive-rejection tests in `tests/schema_apply.rs` now
  assert on the stable code (`msg.contains("OG-DS-104")`) instead of
  the error-message substring. 11/11 tests pass.

- New `docs/schema-lint.md` documents the v0 catalog + the 10 families
  + Atlas prior art. AGENTS.md index updated.

What's explicitly NOT in v0 (subsequent PRs):

- No severity config in `omnigraph.yaml` (MR-694 §2).
- No `@allow(OG-XXX-NNN, "rationale")` suppression directive (§3).
- No `--allow-data-loss` flag or destructive-tier enforcement.
- No new `SchemaMigrationStep` variants (soft/hard drops, default,
  widen/narrow). MR-700, MR-697 land those.
- No pre-migration checks (MR-941).
- No CD / VE / LK / NM family rules (MR-942..945).
- No CI integration (MR-946).

Tests: 235 compiler tests, 11 schema_apply integration tests, 14
lint module tests, 55 CLI tests — all green.

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

AGENTS.md

@@ -73,6 +73,7 @@ Full diagram and concurrency model: [docs/architecture.md](docs/architecture.md)
 | Architecture, L1/L2 framing, concurrency model | [docs/architecture.md](docs/architecture.md) |
 | Storage layout, `__manifest` schema, URI schemes, S3 env vars | [docs/storage.md](docs/storage.md) |
 | `.pg` schema language, types, constraints, annotations, migration planning | [docs/schema-language.md](docs/schema-language.md) |
+| Schema-lint codes (`OG-XXX-NNN`), families, severity, suppression | [docs/schema-lint.md](docs/schema-lint.md) |
 | `.gq` query language, MATCH/RETURN/ORDER, search funcs, mutations, IR ops, lint codes | [docs/query-language.md](docs/query-language.md) |
 | Indexes (BTREE / inverted / vector / graph topology) | [docs/indexes.md](docs/indexes.md) |
 | Embeddings (compiler + engine clients, env vars, `@embed`) | [docs/embeddings.md](docs/embeddings.md) |