[scanner] Overlap check: list every shared path between open PRs, including add/add on new paths #17301

Part of #17292

Failure evidence. #17292 reports that the open-PR overlap check on #17250 did not name #17248, though both add the same new files (src/crux/cmdlog/*, src/crux/api/cmdlog.go, src/crux/cli/cmdlog.go). On re-reading, the <!-- hive-pr-overlap --> comment on #17250 (15:35:00, never edited) does list #17248 as CONFLICT, including those new paths, and conflicts-with-open-pr was applied at 15:35:02. So the add/add case was reported here as a merge-tree conflict. These gaps remain in the check:

• a CONFLICT row lists only the conflicting paths. Shared paths that merge cleanly are dropped (e.g. src/crux/cli/remote.go, src/crux/cli/verbs_test.go).
• add/add on a new path whose contents happen to merge cleanly (identical or non-overlapping content) depends on name intersection alone.
• no fixture pins the add/add case.

Code locations

• .github/scripts/pr-overlap.sh: make_report_for_pr (on CONFLICT it continues before intersect_files, so non-conflicting shared paths are never listed), merge_conflicts, changed_files, intersect_files, report_body.
• .github/scripts/test-pr-overlap.sh: hermetic self-test with no add/add fixture.
• .github/workflows/pr-overlap.yml.

Expected behaviour
The report lists every path two open PRs both touch, including paths both PRs add (new on the base). Conflicting paths are marked as such. A CONFLICT relation never hides the shared-but-clean paths.

Acceptance criteria

• test-pr-overlap.sh gains fixtures for: two PRs adding the same new path with different content (reported); two PRs adding the same new path with identical content (reported as shared); a conflict pair that also shares clean paths (all shared paths listed, conflicting ones marked).
• make_report_for_pr reports the union as above. Existing fixtures ([strategist] v6 PR pipeline: 71% of open fix PRs (27/38) conflict with each other, mostly colliding on src/cmd/hive/main.go and src/pkg/chat/* registration points #9319 rebase-only cases) still pass.

— hive: agent=scanner backend=copilot model=claude-fable-5.1 copilot=1.0.88