fix(guard): honor disabled checks in executor validation

[HaveNiceDa]

Linked Issue Or Context

• Closes hallucinationGuard.enabledChecks is dead config on the executor path #386

Summary

• Make HallucinationGuard fast-path validation honor disabled file existence, syntax, and import checks.
• Add guard-level regression tests plus an executor-path test proving a fully disabled guard no longer blocks create_file validation.

Impact Scope

• Scope is limited to @frontagent/hallucination-guard fast-path validation and focused executor/guard tests.
• No planner, MCP tool, SDD, RAG, Filesense, or security policy behavior changed.

GitNexus Impact Summary

• Risk level: LOW
• Critical skeleton changes: none; touched hallucination guard fast-path methods and tests only.
• GitNexus impact: impact(validateFilePath, upstream) reported LOW with 0 direct upstream symbols, 0 affected processes, 0 affected modules. impact(validateCode, upstream) reported LOW with 1 direct upstream symbol (packages/hallucination-guard/src/guard.test.ts), 0 affected processes, 0 affected modules. detect_changes(scope=all) reported 3 changed files, 3 changed symbols (HallucinationGuard, validateFilePath, validateCode), 0 affected flows, low risk.
• Verification: pnpm agent:bootstrap; HOME=/tmp/frontagent-gitnexus-home pnpm quality:predev; focused guard/core tests; focused typechecks; pnpm quality:precommit; HOME=/tmp/frontagent-gitnexus-home pnpm quality:local; pre-push quality:local hook passed.

Verification

• pnpm --dir packages/hallucination-guard test -- --run src/guard.test.ts
• pnpm --dir packages/core test -- --run src/executor/executor.test.ts
• pnpm --filter @frontagent/hallucination-guard typecheck
• pnpm --filter @frontagent/core typecheck
• pnpm quality:precommit
• HOME=/tmp/frontagent-gitnexus-home pnpm quality:local

Checklist

• I have linked an issue or explained why this PR stands alone.
• I have kept the diff focused on the stated change.
• I have run pnpm quality:precommit, or explained why it could not run.
• I have run pnpm quality:local for critical skeleton changes, or explained why it could not run.
• I have updated docs or tests when behavior, public APIs, or Harness contracts changed.
• For critical skeleton changes, I have filled the GitNexus impact summary with concrete results.