feat(models): commit models.schema.json and validate the SPOT against it in CI (#196)

models.json has declared `"$schema": "./models.schema.json"` since v3.11.0, but
that file was never committed — `git log --all -- models.schema.json` is empty.
So the SPOT that ADR 0003 makes canonical had NO structural validation: a
missing contextWindow, a typo'd openclawName, or a boolean written as a string
would pass every existing test and only surface downstream — in OpenClaw's
registry, or silently inside a truncation budget.

Validated with the repo's OWN validator (lib/structured-output.mjs, shipped for
#153) rather than adding ajv. AGENTS.md requires minimal dependencies and the
repo currently has ZERO; this keeps it that way and exercises that validator on
a second real input. Capability-probed first rather than assumed: it enforces
type / required / enum / const / additionalProperties (both the boolean and the
schema form) / items / minItems, which covers everything the SPOT needs.

Three tests:
  - the $schema reference resolves to a committed file (the #196 bug itself)
  - models.json validates against models.schema.json, strict
  - the schema actually REJECTS 8 corruption shapes — missing required field,
    wrong scalar type, non-integer window, unknown extra field, alias mapped to
    a non-string, empty models array, wrong document version, unknown
    top-level key

That third test is the guard-on-the-guard: without it a vacuous schema (`{}` or
a typo'd `properties`) would make the second test pass on anything.

Mutation-proven in three directions:
  schema replaced with {}              -> "schema failed to reject: missing required field"
  additionalProperties:false removed   -> guard fails (loosened constraint caught)
  models.json corrupted (drop a field) -> strict validation fails

What the schema deliberately does NOT encode: referential integrity — that
every aliases/legacyAliases target exists as a models[].id. That is not a JSON
Schema concept; it is already covered by two dedicated tests, and the schema
says so in its description so the next reader doesn't assume it is covered.

The field descriptions carry the non-obvious consequences that have bitten this
repo before: contextWindow is a GLOBAL lever (max across all entries x 3 sets
MAX_PROMPT_CHARS for every model, ADR 0009), maxTokens is advertising only and
is enforced by OpenClaw rather than OCP, and models[] order is load-bearing
because ocp-connect uses model_ids[0] as a fallback.

Documented per release_kit (new file/SPOT/schema -> contributor section):
AGENTS.md "Key files to know" and README's Available Models section.

Tests: 460 passed, 0 failed (457 + 3).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017gbqUZ8HfBZpjjbzQ85oH8

AGENTS.md

@@ -30,6 +30,7 @@ Runtime: Node.js (ESM, `.mjs` throughout). No build step. No bundler. `server.mj
 
 - `server.mjs` — the proxy itself; every request path lives here. Governed by `ALIGNMENT.md`.
 - `models.json` — single source of truth for model IDs, aliases, and context windows. See ADR 0003.
+- `models.schema.json` — the schema `models.json` declares in its `$schema`. CI validates the SPOT against it (`test-features.mjs`) using the repo's own `validateJsonSchema`, so a malformed entry fails the build instead of surfacing downstream in OpenClaw.
 - `setup.mjs` — first-time installer; reads `models.json` to derive bootstrap config.
 - `scripts/sync-openclaw.mjs` — idempotent OpenClaw registry sync invoked by `ocp update`. See ADR 0004.
 - `ocp` — user-facing CLI (install, update, start, stop, status, logs, etc.).