feat: add --since/--until date filters to workouts list

[marinsalinas]

Summary

• Adds --since / --until to hevy workouts list. Both accept YYYY-MM-DD, both inclusive on the date portion.
• Filtering is client-side — the Hevy /v1/workouts endpoint has no native date params (only /v1/workouts/events does). Applied in both --all (generator) and paginated branches against Workout.start_time.
• Updates the bundled Claude Code skill so the agent knows about the new flags and rewrites the "this week's workouts" example to push filtering into the CLI rather than jq.

Test plan

• make format clean
• make lint / mypy clean (pre-commit hooks confirmed)
• 4 new tests pass: --since excludes older, --until excludes newer, range narrows to one workout, invalid date rejected by BadParameter
• Existing test suite still passes (one pre-existing local-env failure for test_workouts_list_no_api_key due to XDG config leakage — unrelated, separate issue)
• Manual smoke against real account: hevy workouts list --since 2026-05-01 --format json

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.28%. Comparing base (a11b8e3) to head (16721b8).

Files with missing lines	Patch %	Lines
src/hevy_cli/commands/workouts.py	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
+ Coverage   82.02%   82.28%   +0.26%     
==========================================
  Files          14       14              
  Lines        1157     1174      +17     
  Branches      112      115       +3     
==========================================
+ Hits          949      966      +17     
  Misses        176      176              
  Partials       32       32              

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