feat: add context-engine runtime identity diagnostics

[Tosko4]

Summary

• add host-side runtime identity diagnostics for the active context engine
• surface configured engine, active engine, origin/path metadata, host session id, plugin session/conversation ids, and mismatch warnings
• stamp repo-shipped and plugin-registered context engines with explicit origin/path metadata without bundling any external plugin implementation

Why

• operators currently have to guess which context-engine tree is actually live and whether host and plugin session bindings are aligned
• this keeps the Hermes host side generic for external context engines instead of special-casing or vendoring hermes-lcm
• it makes drift and session-boundary mismatches visible without reducing built-in compressor behavior or plugin capabilities

Validation

• pytest tests/run_agent/test_context_engine_runtime_identity.py tests/run_agent/test_plugin_context_engine_init.py tests/agent/test_context_engine.py -q
• python -m compileall -q agent hermes_cli plugins run_agent.py tests/run_agent/test_context_engine_runtime_identity.py
• git diff --check
• local runtime smoke via AIAgent.get_context_engine_runtime_identity() on the live plugin-backed setup

Notes

• scope is host observability/diagnostics only
• no hermes-lcm plugin implementation was vendored or merged into Hermes core in this branch
• runtime smoke covered the current plugin-loaded path; repo-shipped /lcm runtime smoke is not claimed as live-verified in this slice

[Tosko4]

This PR could not be reopened because its original head repository disappeared. Recreated as replacement PR #13373: #13373