feat(observatory): complete evidence-led agent delivery

[jmagar]

Summary

Completes the unraid-mcp-6jfs evidence-fidelity and operator-delivery epic.

• adds canonical Claude, Codex, Gemini, and Antigravity provider descriptors with safe transcript roots and bounded receipt-backed runtime health
• completes authenticated syslog forwarding receipt/recovery coverage and bounded transcript continuation behavior
• hardens Agent Observatory source projection so repository observations, OTLP relationships, cursors, and redacted evidence remain durable and provenance-bound
• exposes the implemented Observatory routes through the audited REST surface and documents the exact MCP/action/schema inventory

Verification

• cargo fmt --check
• git diff --check
• CARGO_BUILD_JOBS=2 cargo test --lib --no-fail-fast -- --test-threads=1
  • 2,664 passed; 0 failed; 1 ignored

Safety notes

OTLP span and metric associations require pre-existing exact provider/session run identity. Untrusted telemetry advances its cursor without creating synthetic run evidence.