feat(schema): reserve validations key in schema spec v0.1.0 (#192)

[zeevdr]

Closes #192. Final Phase-1 subtask of the cross-field validation umbrella (#76); pairs with #193 (`dependentRequired:`) and #194 (prefix-overlap lint), both in main.

Summary

Reserves the top-level `validations:` key in schema spec v0.1.0 — proto, DB, YAML parser, lint, storage round-trip — without shipping the CEL engine. The engine lands in Phase 2 (#76).

Why ship the key now

• Schema Spec v0.1.0 freezes the meta-schema. Adding the key later is a breaking spec bump that downstream tooling has to handle. Reserving now costs ~50 lines of plumbing.
• Schema authors who want cross-field rules today can write CEL expressions; ImportSchema accepts and persists them. They become no-ops at write time until Phase 2.

YAML shape (per .agents/context/cel-validation.md)

```yaml
validations:

• path: payments # optional group prefix (informational in v0.1.0)
  rule: "self.payments.min < self.payments.max"
  message: "min must be less than max"
  severity: error # optional, default error
  reason: MIN_GT_MAX # optional machine code
  ```

v0.1.0 lint at ImportSchema (structural only)

• `rule` and `message` required, non-empty
• `severity` empty or one of `error` / `warning`
• `x-*` extension keys accepted; unknown keys rejected
• Error messages include the index of the offending entry

CEL compilation, field-reference resolution, and contradiction detection defer to Phase 2.

Test plan

• Structural lint — empty rule, empty message, bad severity, unknown extension key, x-* allowed, error includes index.
• Marshal / unmarshal round-trip + empty / malformed JSON edges.
• proto<->YAML converters with order preservation.
• Top-level YAML parser integration — accepts valid block, rejects empty rule, rejects bad severity.
• `make test` passes (full suite, race detector on).
• `make lint` passes.
• `make generate docs` regenerated and committed.