Add bundled Claude Code Agent Skill and installer

[PauBadiaM]

Summary

Adds a bundled Claude Code Agent Skill so coding agents use decoupler correctly, plus a console script to install it.

• Skill (src/decoupler/_skills/data/): a SKILL.md router and a references/ folder of topic files (methods, calling conventions, I/O and output locations, priors, datasets, preprocessing, multi-method consensus, ranking/plotting, benchmarking) that agents read on demand.
• Installer (src/decoupler/_skills/install.py): the decoupler-install-skills console script copies the skill into ~/.claude/skills/decoupler/, making it available across all projects. Use --force to refresh after upgrading, or --print-path to point CLAUDE_SKILLS_PATH at the bundled copy in place. Installation is opt-in rather than happening on import/install.
• Documented in the README and bumped the version to 2.2.0 with a changelog entry.

Tests

tests/test_install_skills.py covers bundled-skill integrity (frontmatter, resolvable internal links), install into a fresh destination, the force overwrite path, FileExistsError/FileNotFoundError handling, exclusion of .ipynb_checkpoints/__pycache__, the default ~/.claude destination, and the --print-path entry point.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.04878% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.85%. Comparing base (ec0e0ee) to head (e71f993).

Files with missing lines	Patch %	Lines
src/decoupler/_skills/install.py	78.04%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #343      +/-   ##
==========================================
- Coverage   94.18%   93.85%   -0.34%     
==========================================
  Files          78       79       +1     
  Lines        4078     4119      +41     
==========================================
+ Hits         3841     3866      +25     
- Misses        237      253      +16     

Files with missing lines	Coverage Δ
src/decoupler/_skills/install.py	78.04% <78.04%> (ø)

... and 2 files with indirect coverage changes

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