[cc] switch JSON Schema generator from anyOf to oneOf for the discriminated union

[anipotts]

Source: Apr 2026 conventions audit

Problem

`lib/action.ts` uses `zod-to-json-schema` with default options, which produces `anyOf` for discriminated unions. `anyOf` matches "at least one branch"; `oneOf` matches "exactly one branch". With const discriminators they're behaviorally equivalent, but `oneOf` is the more accurate signal:

1. JSON Schema 2020-12 spec says discriminated unions SHOULD use `oneOf`
2. OpenAPI tooling (which the model's tool-selection layer may resemble) uses `oneOf`+`discriminator` keyword
3. Some MCP clients may treat `anyOf` permissively (multi-branch match) whereas `oneOf` is exclusive

Fix

```ts
// lib/action.ts
const _raw = zodToJsonSchema(ActionSchema, {
$refStrategy: "none",
target: "openApi3", // emits oneOf with discriminator
});
```

Or post-process the output to rename `anyOf` → `oneOf`.

Acceptance criteria

• tools/list inputSchema uses oneOf (not anyOf)
• Existing tests still pass (action shape unchanged)
• Snapshot test for byte-stable schema (see issue for snapshot test)

[github-actions]

@claude please triage this issue per .github/AI_REVIEW_RUBRIC.md (issue-triage section).