chore: Add take/untake workflow for issue self-assignment

[andygrove]

Summary

• Adds a GitHub Actions workflow that allows contributors to self-assign issues by commenting take and unassign by commenting untake
• Updates the contributor guide to document this feature

Test plan

• Merge to main and test by commenting take on an open issue

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.08%. Comparing base (f09f8af) to head (dc05905).
⚠️ Report is 880 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3270      +/-   ##
============================================
+ Coverage     56.12%   60.08%   +3.95%     
- Complexity      976     1438     +462     
============================================
  Files           119      172      +53     
  Lines         11743    15961    +4218     
  Branches       2251     2641     +390     
============================================
+ Hits           6591     9590    +2999     
- Misses         4012     5040    +1028     
- Partials       1140     1331     +191     

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