Steward: clarify duplicate checks and verification reporting rules in TEST_PLAN.md

[kunkunGames]

This PR updates TEST_PLAN.md with explicit rules derived from recent PR hygiene lessons. It adds fallback instructions for checking duplicate PRs when gh is unavailable (git fetch --all and git branch -a) to prevent duplicate generated-inventory refreshes. It also adds rules to handle environment limitations (stating exact reasons/risks instead of blocking) and truthfully reporting verification steps (only claiming execution if actually run locally).

WorkFingerprint

• Agent: Steward
• Boundary: TEST_PLAN.md
• Primary files: TEST_PLAN.md
• Verification commands and results: git diff --check
• Skipped checks with reasons: No Rust changes, script changes, or policy changes made, so specific commands for those domains were skipped.
• Risk: Zero risk, docs-only change to process guidelines.
• Rollback notes: Revert TEST_PLAN.md to previous state.
• Queue hygiene invariant: Ensure clear instructions exist for fallback duplicate checking and truthful test reporting.
• Related PRs/issues checked: Open PRs reviewed; no direct overlaps with this documentation change.
• Why this is non-overlapping: Focuses explicitly on text clarification in the test plan, completely decoupled from active feature work or inventory refreshes.

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PR created automatically by Jules for task 6481747454428583527 started by @kunkunGames

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