feat: fail when global telemetry limits cannot be enforced

[brandonwise]

Summary

This PR tightens release safety: if global average telemetry limits are configured (max_avg_latency_ms / max_avg_cost_usd) but telemetry is missing, the gate now fails instead of silently passing.

Why

Teams are using agent quality gates to stop regressions in CI, but missing telemetry created a blind spot where configured global limits were never enforced.

What changed

• Updated evaluator logic to fail when:
  • max_avg_latency_ms is set and no latency telemetry is present
  • max_avg_cost_usd is set and no cost telemetry is present
• Added focused tests for both new fail-fast paths
• Updated README to document telemetry-required behavior for global limits

Validation evidence

All checks run locally and passed.

1. Repo-level/full test suite

• Command: python3 -m pytest
• Result: ✅ pass (7 passed in 0.04s)

2. Targeted tests for changed module

• Command: python3 -m pytest tests/test_evaluator.py -k 'global_latency_limit_requires_telemetry_when_configured or global_cost_limit_requires_telemetry_when_configured'
• Result: ✅ pass (2 passed, 4 deselected in 0.01s)

3. Lint/type/build checks

• Command: python3 -m ruff check src tests
• Result: ✅ pass (All checks passed!)
• Command: python3 -m build
• Result: ✅ pass (Successfully built ... .tar.gz and ... .whl)

4. Smoke/integration check

• Command: PYTHONPATH=src python3 -m agent_release_gate.cli evaluate --spec examples/spec.yaml --results examples/results.json
• Result: ✅ pass (gate_passed=True, pass_rate=1.0)

Risk

Low. Change is scoped to gate decision logic for already-configured global limits.