feat: optional secrets preset and snapshot-bound review gates

[renezander030]

Adds skillgate init --preset no-secrets, an optional TruffleHog credential gate with a local infrastructure-name denylist, and an optional snapshot-bound OCR/delegated-review report gate. TruffleHog and OCR stay external; scanner output is redacted, failures block, and these gates always re-evaluate even with --cache. Prepares v0.12.0 metadata and documentation.

The delegated OCR thin slice on real PR #39 reproduced an invalid-policy bypass: opencode allowed a non-shell publish tool when it could not load the policy. The plugin now blocks unknown/MCP tools in that case while preserving built-in repair tools. The historical implementation allows the fixture; the fixed implementation blocks it.

Review reports are explicitly trusted reviewer attestations, not independent proofs that review ran. They must be complete, have no findings/warnings, and match policy, working files, and staged blob IDs. Verified-only credential detection also depends on provider reachability; static pattern gates remain useful for unsupported/unverifiable secrets.

Validation:

• 168 tests passed; Node 22 coverage: 93.54% lines, 80.75% branches, 97.74% functions.
• All four repository self-gates passed; npm package dry run includes both runtime modules and the regression example.
• Pinned TruffleHog v3.97.9: clean native-gate smoke passed in 5.4s; staged public vendor canary blocked in 4.2s with a redacted receipt.
• Added failure-path tests for scanner availability/errors/timeouts/redaction, staged infrastructure names, review freshness, preset initialization, and invalid-policy publish tools.