Update GitHub issue and PR templates

[kmcginnes]

Description

Overhaul the GitHub issue templates and PR template for consistency, clarity, and to support issue types.

• Clean up front matter: fix labels syntax, remove unused fields (title, assignees, projects), add type field
• Improve about descriptions to be concise and help users pick the right template
• Add new templates for Epic, Task, and Spike issue types
• Add config.yml to disable blank issues
• Simplify template bodies: remove low-value sections (Additional Context, Tasks), streamline callout blocks
• Update Related Issues comments to guide users toward bulleted lists for auto-expanded titles
• Update PR template checklist: clarify pnpm checks and pnpm test items, replace changelog item with documentation item
• Update GitHub skill to map issue types to their corresponding templates

Validation

Create a new issue on GitHub and verify the template picker shows all five types in order with correct descriptions. Verify blank issues are disabled.

Related Issues

• Depends on Rename issue templates for consistent ordering #1591

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.90%. Comparing base (c5affc5) to head (a2a6c97).
⚠️ Report is 101 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1592       +/-   ##
===========================================
+ Coverage   47.81%   64.90%   +17.08%     
===========================================
  Files         382      372       -10     
  Lines        8525     8382      -143     
  Branches     3159     3123       -36     
===========================================
+ Hits         4076     5440     +1364     
+ Misses       3070     2087      -983     
+ Partials     1379      855      -524     

Flag	Coverage Δ
unittests	64.90% <ø> (+17.08%)	⬆️

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