docs: Add implementation plan for xdist flaky detection support

[remyduthu]

Refs: MRGFY-6296

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

Depends-On: #381

[remyduthu]

This pull request is part of a stack:

1. docs: Add design spec for pytest-xdist flaky detection support (#380)
2. docs: Address spec review feedback for xdist flaky detection (#381)
3. docs: Add implementation plan for xdist flaky detection support (#382) 👈
4. feat(flaky-detection): Add pytest-xdist support to flaky detection system (#383)

[mergify]

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