Declare Make targets as phony

[anth-volk]

Fixes #2591

Summary

• declare every current command-only Make target as phony
• require AI-authored Makefile changes to keep command-only targets in .PHONY
• add the canonical guidance and lookup pointers for Codex, Claude, and Copilot
• add a changelog fragment describing the Make behavior

Verification

• make -n install setup-env debug debug-asgi test-env-vars test quality-guards typecheck-v2 debug-test format changelog
• uv run --frozen towncrier build --draft
• git diff --check github/master...HEAD

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.18%. Comparing base (37e168a) to head (6e0502b).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3822       +/-   ##
===========================================
+ Coverage   46.26%   87.18%   +40.92%     
===========================================
  Files         135      150       +15     
  Lines        7963     8935      +972     
  Branches     1326     1542      +216     
===========================================
+ Hits         3684     7790     +4106     
+ Misses       3990      703     -3287     
- Partials      289      442      +153     

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[anth-volk]

Deployment validation for this PR currently depends on resolving the pre-existing failure in tests/integration/test_live_phase10_staging.py::test_live_phase10_activation_failure_and_retry on master (failed workflow run). This PR should remain blocked until the existing deployment sequence completes successfully.