Add read-only local backend metadata probe (CR-114)

[coreytshaffer]

Summary

Adds CR-114: a read-only local backend metadata probe for Ollama, LM Studio, and llama.cpp-compatible local endpoints.

The probe records metadata only. It does not generate model output, call completions, call embeddings, write ledger events, feed routing, or change ResilienceRouteInput.

Scope

• New riage_core/local_backend_probe.py
• New c probe CLI surface
• New offline probe tests
• CR-114 request document
• Local backend telemetry doc updated to state that CR-114 implements the probe slice only

Non-goals

• No routing changes
• No route-input population
• No circuit breakers
• No degraded modes
• No background polling
• No auto-discovery
• No model generation
• No completions or embeddings
• No cloud/frontier probing
• No ledger writes
• No default artifact writes

Validation

• python -m pytest -q tests/test_local_backend_probe.py -> 18 passed
• c audit --privacy-invariants -> passed, 698 records clean
• scoped diff check passed on the five approved files
• cached file list matched exactly the five approved files