fix(ladon): use native structured review output

[bokelley]

Summary

• replace the reviewer's best-effort findings-file Write with Claude Code Action's native --json-schema output
• pass schema-validated findings directly to the arbiter and keep missing output fail-closed
• remove filesystem write permission from the reviewer
• add a manifest regression test for the structured-output contract

This fixes the repeated Reviewer did not write a findings file failures seen on adcontextprotocol/adcp#6794 after otherwise successful reviews.

Verification

• npm run test (155 tests)
• isolated TypeScript checks for all three workspaces
• direct ncc builds for setup, arbiter, and versioning actions
• Prettier/Biome and git diff --check