feat: add fastsqla-testing agent skill

[hadrien]

Problem

• AI agents lack context on FastSQLA testing patterns, especially the fixture chain and teardown requirements

Solution

• Add an Agent Skill (SKILL.md) covering pytest fixture chains, ASGI integration testing, and critical teardown patterns

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6e98cf4) to head (a6ae5aa).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #32   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines          115       115           
  Branches         7         7           
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  Hits           115       115           

Flag	Coverage Δ
fastapi-post121	100.00% <ø> (ø)
fastapi-pre121	100.00% <ø> (ø)
sqlmodel	100.00% <ø> (ø)

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[hadrien]

Suggested change

	name: fastsqla-testing
	name: fastsqla-sqlite-testing

Seems very sqlite specific.