feat: implement strict spec maintenance with specs.refresh RPC (closes #620)

[alexhraber]

Summary

Implements Issue #620 - Researching Spec Management

Changes

1. src/core/project_specs.rs - Added function that:

   • Recalculates content hashes for all spec files
   • Updates
   • Updates timestamp
   • Writes the refreshed manifest back to disk

2. src/core/rpc.rs - Added:

   • and types
   • capability registration

3. src/core/validate.rs - Modified :

   • NEW: Fails with if spec content hash doesn't match manifest
   • CHANGED: Changed stale fingerprint from to with

4. src/lib.rs - Added:

   • RPC handler
   • operation dispatch

Verification

The issue mentions these tests:

• Fingerprint Failure: Modify a codebase file and verify decapod validate fails
• Content Hash Failure: Modify a spec file and verify decapod validate fails
• Refresh Success: Run and verify decapod validate passes again

See Issue #620 for full details.