Remove ignoredusers from config template and help text

[clcollins]

Summary

Follow-up to #276. Now that ignored users are auto-discovered from silent escalation policies, remove ignoredusers from:

• The example config generated by config --create
• The optionalKeys help map used by config --validate

The ignoredusers key is still accepted at runtime (backward compatible via the deprecation path in #276), but new configs will no longer include it.

Test plan

• go test ./... passes
• config --create no longer includes ignoredusers in generated config
• config --validate no longer lists ignoredusers as an optional key

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.56%. Comparing base (bd30bb8) to head (71fd4a9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #277   +/-   ##
=======================================
  Coverage   67.56%   67.56%           
=======================================
  Files          34       34           
  Lines        5210     5210           
=======================================
  Hits         3520     3520           
  Misses       1470     1470           
  Partials      220      220           

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