refactor: replace HTTP server with file-based metrics, require GitHub token

[Copilot]

Replaces HTTP server architecture with file-based metrics storage and removes manual step marker support. GitHub token now required for automatic step tracking.

Changes

Architecture shift: HTTP → File-based

• Removed server.ts (HTTP server on port 7777, REST endpoints for metrics/markers)
• Added collector.ts (background process writing to temp file)
• Metrics written to /tmp/metrics-{GITHUB_RUN_ID}-{GITHUB_JOB}.json after each collection cycle
• Eliminates port conflicts, server readiness checks, and network stack dependency

Removed manual step markers

• Deleted mark-step.ts CLI tool and Metrics.markStep() method
• Removed manual marker merge logic in post action
• Step tracking now exclusively via GitHub API

Made github-token required

• action.yml: changed input from optional to required
• fetchWorkflowSteps(): throws if token missing (was silent fallback to empty array)
• Post action always uses API-fetched steps

Migration

# Before (optional token, manual markers supported)
- uses: garbee/actions-workflow-metrics@v1

# After (required token, automatic tracking only)
- uses: garbee/actions-workflow-metrics@v1
  with:
    github-token: ${{ secrets.GITHUB_TOKEN }}

Stats

• 19 files changed: +324, -573 lines
• Removed get-port dependency
• Reduced bundle size ~518KB (removed mark-step bundle)
• 29 tests passing

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