feat: infer writable OpenAPI operations

[sabino]

Stacked on #12.

What changed

• add IMPORT FOREIGN SCHEMA ... OPTIONS (writable 'true')
• pair GET collections with POST and one-identity item PATCH/PUT/DELETE operations
• infer row identity, request-only columns, readOnly exclusions, JSON names, and body whitelists from OpenAPI request schemas
• keep operations read-only whenever a safe identity/body contract cannot be inferred
• add a control-plane opt-in and display each discovered table's INSERT/UPDATE/DELETE capabilities
• preserve the writable flag in redacted portable bundles and SQL previews
• ship a reviewable GoRest.in CRUD contract and a manual disposable-user validation where SQL performs INSERT/GET/PATCH/PUT/DELETE with unconditional API-side cleanup
• keep the default HTTP User-Agent synchronized with the package version
• release the stack as v0.4.0 with a 0.3.2 to 0.4.0 extension upgrade path

Why

The native mutation callbacks in #12 are deliberately explicit. This layer makes them easy to configure from a compatible OpenAPI document without hiding the generated PostgreSQL contract or guessing when an API is ambiguous.

Validation

• deterministic real-PostgreSQL CRUD added to the PG14-18 matrix
• control-plane discovery/import/export integration extended for writable sources
• live GoRest.in probe confirmed generated-ID recovery through the documented email filter and complete cleanup
• cargo fmt --all -- --check
• Python, JavaScript, shell, YAML, native package, and native installer checks
• full native and control-plane builds delegated to this PR's GitHub Actions under local disk pressure

Important boundary

HTTP side effects cannot be rolled back with PostgreSQL. Multi-row writes are per-row and can partially succeed. POST/PATCH are not retried automatically, and the current callback adapter does not support RETURNING.