Add distributable tpcp skills installer and multiprocessing docs

[AKuederle]

Summary

• add a packaged CLI that copies bundled user-facing skills into a project's folder
• move repo-local skills to and ship user-facing tpcp skills from a top-level directory
• add multiprocessing caveats documentation and link it from the relevant docs entry points
• fix related API doc inconsistencies and document the new skills install command in the README

Validation

• installed the built wheel into a fresh temp venv and ran successfully

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.00000% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.67%. Comparing base (13d7e71) to head (362d7f9).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/tpcp/_cli.py	76.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
- Coverage   78.72%   78.67%   -0.06%     
==========================================
  Files          28       29       +1     
  Lines        2463     2513      +50     
==========================================
+ Hits         1939     1977      +38     
- Misses        524      536      +12     

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