Improve repo-assist: regression test verification, windows-only revisit, stricter labeling (#19499)

- Enable create-pull-request and push-to-pull-request-branch for regression test PRs
- Enable remove-labels for AI-thinks-issue-fixed and AI-thinks-windows-only
- Add Task 2: Regression Test Verification with adversarial dispute step
- Add Task 3: Systematic revisit of AI-thinks-windows-only claims
- Reorder tasks: Task 1 → Task 3 → Task 2 → FINAL (Task 3 feeds into Task 2)
- Strict windows-only rules with explicit FCS-testable feature list
- Fix memory section to match actual state.json schema
- Remove close-issue actions (closure happens via PR merge with Fixes link)
- Fix stale references, contradictory instructions, date cutoff to 2024
- Clarify anti-spam rules for multi-task commenting

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: abonie <20281641+abonie@users.noreply.github.com>

Author: 3 people