Fix secure validation for fork skill submissions

[denial123789]

What changed

• Materialize a fork PR head with trusted git fetch/git worktree operations instead of asking actions/checkout to opt into unsafe checkout under pull_request_target.
• Run the base branch's trusted validator against the untrusted ./pr data directory, rather than executing a fork-modified validator.
• Diff the fetched PR ref from the trusted checkout so changed skill directories are detected correctly.
• Remove the stray sks identifier at the end of scripts/skills-security-scan.mjs.

Why

Fork submissions currently stop before validation because actions/checkout rejects an untrusted fork checkout in a privileged pull_request_target run. Later steps then obscure the root cause with missing-report errors. Independently, the scanner writes its report and then throws:

ReferenceError: sks is not defined

The new flow keeps executable workflow and validation code on the trusted base revision. Fork content is materialized only as input data; the validator already rejects symlinks and unsafe file types.

Validation

• YAML parsing for both changed workflow files
• simulated fork diff detected multi-source-search
• trusted base validator executed against the fork worktree — passed
• trusted base scanner executed with --repo-root against the fork worktree — FAIL=0
• node scripts/skills-security-scan.mjs — completed with a WARN report and no runtime exception
• node scripts/validate-skills.mjs — 7 base skill directories validated
• git diff --check

Closes #59.