feat(metrics): lightweight read-only GET /metrics raw-signal endpoint (ORCH-099)

FND/F1a: add a versioned read-only JSON endpoint GET /metrics that exposes the
orchestrator's own raw state for the future observability sidecar F1b — active
task stages, job queue, agent-liveness (pid/runtime/cpu_ticks), and cost/tokens.
The orchestrator emits ONLY raw signal it alone knows; thresholds/alerts/history
live in the separate sidecar (observer separated from observed, BRD §1).

- src/metrics.py: new leaf collector build_metrics() (never-raise per section,
  serial_gate.snapshot() pattern); envelope schema_version/generated_at/clk_tck +
  stages/queue/agents/cost. _read_cpu_ticks(pid) reads utime+stime from
  /proc/<pid>/stat (null on None/dead/non-Linux pid — never raises).
- src/main.py: thin @app.get("/metrics") wrapper (style of GET /queue).
- src/db.py: read-only helpers get_running_agents() (dedicated SELECT, not an
  extension of the hot-path get_running_jobs()), agent_cost_totals(),
  queue_retry_stats(); job_status_counts() default dict gains the cancelled key.
- src/config.py: metrics_endpoint_enabled kill-switch (default True), env
  ORCH_METRICS_ENABLED via explicit validation_alias so the documented switch
  actually controls the flag.
- docs: README API table row + CHANGELOG entry (contract section already added
  by architect); .env.example ORCH_METRICS_ENABLED.

Strictly read-only / never-raise: STAGE_TRANSITIONS / QG_CHECKS / check_* /
machine-verdict keys / DB schema untouched; /health//status//queue byte-for-byte.
Tests: tests/test_metrics.py (TC-01..TC-11) + env-alias tests in test_config.py.
Full suite green (1482).

Refs: ORCH-099
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -394,6 +394,12 @@ ORCH_COVERAGE_EPSILON=0.5
 ORCH_COVERAGE_TOOL_FAIL_CLOSED=false
 ORCH_COVERAGE_RUN_TIMEOUT_S=900
 
+# ORCH-099 (FND/F1a): operator off-switch for the read-only GET /metrics endpoint
+# (raw-signal snapshot for the F1b sidecar). Default true -> available out of the
+# box. false -> /metrics returns a minimal parsable body {"schema_version":1,
+# "enabled":false} (200, not 404). The endpoint is inert / read-only anyway.
+ORCH_METRICS_ENABLED=true
+
 # ORCH-021: post-deploy production monitoring + degradation reaction. After the
 # terminal deploy->done transition for an applicable repo, a reserved-agent job
 # `post-deploy-monitor` (no LLM, modelled on deploy-finalizer) probes prod over a