v0.5.2

Witness Fabric v1 + optional dependency hardening.

Highlights:

• Add Canonical Witness Envelope spec (cwe_v1)
• Add Ed25519 keystore + witness signing/verification modules
• Add patch-id stable computation in git bridge
• Add CLI surfaces: agentmesh witness verify, agentmesh key generate/list
• Add witness optional dependency extra: agentmesh-core[witness]
• Harden fallback: core installs without cryptography still support agentmesh commit via episode trailer
• Add regression tests for optional dependency behavior

Validation:

• 136 tests passing
• CI matrix + package-smoke green