feat(memory): MemoryProvider.health_check() + Honcho/Mem0 native probes

[PowerCreek]

Summary

• Adds health_check() -> tuple[bool, str] to MemoryProvider ABC with a safe default that delegates to is_available(); returns typed prefix reasons (auth:, unreachable:, unavailable:)
• Gives HonchoMemoryProvider a native override that calls get_honcho_client() for a real connectivity probe
• Gives Mem0MemoryProvider a native override that calls client.get_all(limit=1) to verify key + API reachability
• Collapses the Honcho-specific and Mem0-specific branches in hermes_cli/doctor.py into a single generic path that calls provider.health_check() — the Honcho branch retains config-file existence and enabled checks before delegating to the provider

Test plan

• hermes doctor with Honcho configured + reachable: shows "Honcho connected workspace=..."
• hermes doctor with Honcho configured + bad key: shows auth-rejected issue
• hermes doctor with Honcho configured + unreachable host: shows connection-failed issue
• hermes doctor with Mem0 configured + valid key: shows "mem0 reachable"
• hermes doctor with Mem0 configured + no key: shows auth issue
• hermes doctor with a third-party provider (e.g. openviking): goes through generic else path, calls health_check() (default delegation)
• MemoryProvider.health_check() base default: returns (False, "unavailable: is_available() returned False") for a provider whose is_available() returns False

Closes #42

🤖 Generated with Claude Code

[PowerCreek]

Closing as superseded. The full RFC #42 surface was shipped as eight incremental PRs by hermes-maintainer over the same session (one per provider migration + doctor branch collapse): #48 mem0, #49 byterover, #50 retaindb, #51 (skipped), #52 holographic + Honcho/Mem0 elif collapse, #53 ACP installation probe (separate doctor work), #54 web toolset hint (separate doctor work). RFC #42 is closed; this PR has no orphan changes. — orchestrator cleanup.