feat: Markdown Link Validator

[eli64s]

PR: Markdown Link Validation Feature

Overview

This PR adds a new feature to validate all links within a markdown document.

Key Changes

• Added MarkdownLinkValidator to check link validity
• Integrated link validation into CLI with --validate-links option
• Updated documentation to showcase new link validation feature
• Add requests to project dependencies for link checking
• Updated CI/CD workflow to support Python 3.13

New CLI Feature

Validate links in markdown files:

splitme-ai --validate-links.i README.md

This will:

• Check all links in the markdown file
• Report broken or inaccessible links
• Provide a summary of link validation results

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 36.84211% with 108 lines in your changes missing coverage. Please review.

Project coverage is 37.14%. Comparing base (24d4c62) to head (6543fb5).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/splitme_ai/tools/markdown_link_validator.py	20.83%	57 Missing ⚠️
src/splitme_ai/cli.py	52.30%	31 Missing ⚠️
src/splitme_ai/tools/reference_link_extractor.py	36.66%	19 Missing ⚠️
src/splitme_ai/core.py	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
- Coverage   39.73%   37.14%   -2.60%     
==========================================
  Files          10       11       +1     
  Lines         531      630      +99     
==========================================
+ Hits          211      234      +23     
- Misses        320      396      +76     

Flag	Coverage Δ
unittests-3.10	37.14% <36.84%> (-2.60%)	⬇️
unittests-3.11	37.14% <36.84%> (-2.60%)	⬇️
unittests-3.12	37.14% <36.84%> (-2.60%)	⬇️
unittests-3.13	37.14% <36.84%> (?)
unittests-3.9	37.14% <36.84%> (-2.60%)	⬇️

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