fix(plugins): reject unknown filename directives

[nightcityblade]

Summary

• validate parsed filename directives against the existing allowlist
• continue accepting dynamic <target>-only directives and trailing tilde fences
• add regression coverage for typo rejection and update the collision fixture to use a valid directive

Why

KNOWN_DIRECTIVES was defined but never consulted, so a typo such as ~overwrit was silently carried into the VFS as an inert condition. Failing fast with the filename and bad token prevents broken plugin bundles from shipping while preserving the repository's supported target-only and fenced filename forms.

Validation

• npm --prefix src/rosettify-plugins test -- tests/unit/vfs/directives.test.ts --reporter=verbose — passed
• ROSETTIFY_PLUGINS_LOG_LEVEL=warn npm --prefix src/rosettify-plugins test -- --reporter=minimal — 574 passed
• npm --prefix src/rosettify-plugins run typecheck — passed
• npm --prefix src/rosettify-plugins run build — passed
• ROSETTIFY_PLUGINS_LOG_LEVEL=warn venv/bin/python scripts/pre_commit.py — passed

Checklist

• Scope is narrow and explicit
• Backward-compatible directive forms are preserved
• New behavior has regression coverage
• Local validation passes
• Commit includes the required DCO sign-off

AI assistance was used to inspect the issue, implement the focused change, and run validation. I reviewed every changed line.

Closes #272