fix: remove double usage_count increment in Session.retire()

[vdusek]

Summary

• Session.retire() incremented _usage_count, but callers (mark_good(), mark_bad()) already increment it before calling retire(), causing a double-increment when a session is retired at the usage limit.
• Removed the redundant _usage_count += 1 from retire().
• Added tests covering the no-double-increment invariant for mark_good(), mark_bad(), and direct retire() calls.

Test plan

• Existing session tests pass
• New tests verify usage_count is incremented exactly once per mark_good()/mark_bad() call, even when retirement is triggered

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.24%. Comparing base (02a18ea) to head (3d72b64).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1816   +/-   ##
=======================================
  Coverage   92.23%   92.24%           
=======================================
  Files         157      157           
  Lines       10863    10881   +18     
=======================================
+ Hits        10020    10037   +17     
- Misses        843      844    +1     

Flag	Coverage Δ
unit	92.24% <ø> (+<0.01%)	⬆️

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[Mantisus]

LGTM