feat: add org.openedx.frontend.layout.header_discussions.v1 plugin slot

[brian-smith-tcril]

Summary

• Adds a new org.openedx.frontend.layout.header_discussions.v1 plugin slot wrapping LearningHeader so the discussions header can be wrapped, modified, or replaced via the plugin framework — mirroring the HeaderSlot pattern in frontend-app-learning.
• Wires the new HeaderSlot into DiscussionsHome in place of the direct LearningHeader import.
• Implemented in TypeScript: prop types live on a HeaderSlotProps interface and defaults use ES6 default parameters instead of PropTypes/defaultProps.
• Adds slot README (with border and replace examples + screenshots) and registers the slot in src/plugin-slots/README.md.

Closes #857

Test plan

• npm run lint passes
• npx tsc --noEmit passes
• npm test passes
• Manually verify the discussions header still renders unchanged with no plugin configured
• Manually verify a Wrap plugin against the slot renders around the default header (border example)
• Manually verify a keepDefault: false + Insert plugin fully replaces the header (custom example)

🤖 Generated with Claude Code (I copied the .jsx HeaderSlot component from frontend-app-learning and had claude convert the proptypes to typescript)

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.57%. Comparing base (f64631c) to head (1925324).

Files with missing lines	Patch %	Lines
src/plugin-slots/HeaderSlot/index.tsx	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #868      +/-   ##
==========================================
- Coverage   94.64%   94.57%   -0.07%     
==========================================
  Files         164      165       +1     
  Lines        3623     3632       +9     
  Branches      988      997       +9     
==========================================
+ Hits         3429     3435       +6     
- Misses        181      184       +3     
  Partials       13       13              

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