feat: add metrics-coverage analyzer

[marcus]

Summary

• Adds nightshift metrics-coverage [path] command that scans Go source files using go/ast to detect metrics instrumentation patterns (Prometheus, OpenTelemetry, StatsD, expvar, custom counters/gauges)
• Computes per-package coverage (ratio of instrumented exported functions to total), identifies gaps (uninstrumented HTTP handlers, error paths, public API functions), and assigns risk levels (critical/high/medium/low)
• Results render as markdown or JSON (--json), with optional SQLite persistence (--save) via new migration v6 (metrics_coverage_results table)

Test plan

• Unit tests for scanner with various Go source strings (no files, no exports, uninstrumented, Prometheus, expvar, HTTP handlers, error returns, custom keywords, multiple packages)
• Unit tests for coverage summary computation and risk assessment
• Unit tests for report generation and markdown rendering (sections, gaps, no-gaps case, recommendations)
• All existing tests continue to pass (migration v6 applies cleanly)
• gofmt, go vet, go build pass cleanly

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