Open PRs carry no file-path metadata, so the collision engine misses open-vs-open path overlap

[JSONbored]

Summary

PullRequestRecord (the type for OPEN PRs) has no changedFiles field, unlike
RecentMergedPullRequestRecord, which does. prItem() in src/signals/engine.ts (which converts an
open PR into a CollisionItem for the collision/related-work engine) never populates changedFiles,
while recentMergedItem() does.

The only place file paths are genuinely compared today, buildPreStartCheck(), is explicitly scoped to
merged history only — its own code comment says "Open PR records carry no file metadata in the
cache." It's wired only to an opt-in MCP tool and API route, never invoked automatically by the webhook
review pipeline.

The live per-PR review path (src/queue/processors.ts) calls buildCollisionReport() with no
recentMergedPullRequests argument and builds a buildPreflightResult() input that omits
changedFiles, even though the PR's diff has often already been resolved a few lines earlier for the
secret-scan/AI-review gates. The data model already has what's needed — the pull_request_files table
is populated per open PR by the routine detail-sync backfill, and listRepoPullRequestFilePaths can
already query file paths across an arbitrary PR-number set for a repo.

Net effect: two independently-open PRs touching the same file get zero warning from Gittensory's
"Related work" panel today — it reads "No same-issue or scoped active PR overlap found" for exactly
this case, wasting review cycles on work that will conflict.

Fix

Populate changedFiles on PullRequestRecord/prItem() from the pull_request_files cache so the
existing termOverlap()/collision-scoring machinery (which already tokenizes changedFiles for merged
PRs) gets real path signal for open-vs-open PR pairs too — not just title/label/linked-issue text.

Guarded by a same-author exclusion: a contributor's own two PRs sharing a file (follow-up / stacked PR)
must not be flagged as a collision — that's legitimate iteration, not duplicate work. The guard is
scoped specifically to matches driven by path terms; pre-existing title/label-overlap behavior between
same-author items (already shipped, already tested) is unchanged.

Flag-gated (GITTENSORY_OPEN_PR_FILE_COLLISION, default OFF) for the live-wiring rollout since this
touches the shared collision/preflight path several other gates depend on.

[linear-code]

JSO-2515