ci: skip FP stability and convergence when no relevant files changed

[sbryngelson]

Summary

• Adds a file-changes job (dorny/paths-filter) to both fp-stability.yml and convergence.yml
• Main job is gated: skips unless checkall is true (src/tests/scripts changed) or event is workflow_dispatch
• Registers both workflow files in file-filter.yml so edits to them trigger a run
• Also aligns convergence.yml pull_request trigger to use explicit types: like the other workflows

Test plan

• Open a PR that only touches docs/CI-unrelated files — both workflows should show "Detect File Changes" passing and main job skipped
• Open a PR that touches a .fpp file — both workflows should run normally

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.95%. Comparing base (2ecf477) to head (ce848b2).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1425   +/-   ##
=======================================
  Coverage   64.95%   64.95%           
=======================================
  Files          72       72           
  Lines       18880    18880           
  Branches     1573     1573           
=======================================
  Hits        12263    12263           
  Misses       5641     5641           
  Partials      976      976           

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