docs: define testing approach for agents

[LogenNineFingersIsAlive]

Summary\n- add a dedicated Testing Approach (Required for Agents) section to AGENTS.md\n- codify principles for readable, requirement-led tests\n- add explicit guardrails against gaming tests and shipping flaky-test hacks\n- require hermetic integration tests (isolated DB/env + cleanup)\n- clarify placeholder-test policy on feature branches and before merge\n\n## Notes\n- docs-only change\n

[Copilot]

Pull request overview

Adds explicit, agent-facing testing guidelines to ensure changes are validated with readable, requirement-led tests and to discourage flaky-test or “green-at-all-costs” practices.

Changes:

• Introduces a new Testing Approach (Required for Agents) section in AGENTS.md.
• Defines principles for readable tests, requirement-driven coverage, and red→green→refactor workflow.
• Sets guardrails for flaky tests and requires hermetic integration tests (isolated DB/env + cleanup).

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[Copilot]

This line introduces a curly quote character (“ ”). The rest of AGENTS.md appears to use plain ASCII punctuation, so keeping straight quotes avoids encoding/grep/copy-paste issues in terminals and tooling.

Suggested change

	3. **Do not “game” tests.**
	3. **Do not "game" tests.**

[Copilot]

This heading uses a Unicode arrow (→). Consider using ASCII (e.g., "->" or words like "then") to keep the document consistent and avoid display issues in some environments.

Suggested change

	4. **Default to red → green → refactor.**
	4. **Default to red -> green -> refactor.**