Add broken-link validation with Web Archive fallback

[konard]

Summary

• add a dedicated Broken Link Checker workflow for Markdown and HTML changes
• run lychee without immediately failing so broken URLs can be checked against the Wayback Machine
• add a Python fallback helper that emits actionable notices/errors and rejects links with no archived snapshot
• exclude docs/case-studies references while retaining bounded runtime, least-privilege permissions, and per-job cancellation

Reproduction

On main, adding [dead](https://example.com/definitely-not-a-page-404) to README.md does not start any link-validation workflow, so the dead URL can pass CI.

Verification

• pytest tests/test_check_web_archive.py tests/test_workflows.py -q
• ruff check .
• ruff format --check .
• mypy src/
• python scripts/check_file_size.py
• pytest --cov=src --cov-report=term (69 passed, 100% package coverage)
• npx --yes -p secretlint -p @secretlint/secretlint-rule-preset-recommend secretlint '**/*'

The regression tests verify lychee report parsing, Wayback snapshot handling, conservative API-error behavior, workflow triggers, exclusions, permissions, timeout/concurrency, and the failure gate.

Fixes #49

[konard]

Working session summary

Implemented and finalized PR #52.

• Added Markdown/HTML link checking with lychee.
• Added Python Wayback Machine fallback with actionable annotations.
• Added regression tests and changelog fragment.
• All 69 local tests and quality checks pass.
• Fresh GitHub CI, security scans, and the new link checker pass.
• Branch is clean, current with main, and PR is ready for review.

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[konard]

🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

• Model: GPT-5.6 Sol
• Provider: OpenAI
• Public pricing estimate: $2.387843

📊 Context and tokens usage:

• 77.2K / 200K (39%) input tokens, 10.2K / 128K (8%) output tokens

Total: (77.2K + 1.2M cached) input tokens, 10.2K output tokens, $2.387843 cost

🤖 Models used:

• Tool: OpenAI Codex
• Requested: gpt-5.6-sol
• Thinking level: low (~7999 tokens)
• Model: GPT 5.6 Sol (gpt-5.6-sol)

📎 Log file uploaded as Gist (950KB)

• View complete solution draft log

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Now working session is ended, feel free to review and add any feedback on the solution draft.

[konard]

🎉 Auto-merged

This pull request has been automatically merged by hive-mind.

• All CI checks have passed

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Auto-merged by hive-mind with --auto-merge flag