Route schema validation without context waste

[raghubetina]

What

• treats the bundled Foundation Plan 0.19 schema as machine-readable validator input rather than prose
• uses only a user-named or project-exposed JSON Schema 2020-12 command after confirming it exists
• forbids broad validator discovery, fetch-and-run tools, automatic installation, and improvised validators
• preserves truthful validation, server-acceptance, identity, and product-meaning boundaries
• adds harness-neutral evals for absent, named, failing, and library-only validator paths

Why

Cross-repository dogfood showed fresh agents reading the full 4,961-line schema and probing several language ecosystems when no validator was installed. This keeps authoring productive from the narrative references while still using the exact schema when a compatible command is already available.

Checks

• sh script/check — 11/11 passing
• npm audit --audit-level=low — 0 vulnerabilities
• gh skill publish --dry-run — passed with the existing license/tag-protection advisories
• bundled schema SHA-256 unchanged: 5994c41f65eab52f92020fa24437e76b6957b7016ccf231dce06e8097f0b34b5
• git diff --check

[raghubetina]

Stacked follow-up #4 is now open: #4

It keeps this PRs validator routing unchanged and adds the next bounded authoring capability: enum Fields with ordered, identity-bearing values and optional ordinality, matched to firstdraft/firstdraft#196. The bundled schema digest and CLI surface remain unchanged.

[raghubetina]

This slice has landed on main through #6, which integrated the complete reviewed Skill stack. This PR remains the focused review history for schema-validation routing; no implementation was discarded. Closing as superseded by #6.