fix(pipeline): exclude deleted files from reviewer coverage

[piekstra]

Problem

A PR that deletes files can never be approved by cr — it always posts as a comment, even with zero findings.

Chain:

1. Deleted paths are in the diff, so they're assigned to a reviewer.
2. A reviewer can't inspect a file that no longer exists at head, so it reports the path skipped.
3. buildReviewerCoverage marks that reviewer incomplete_skipped (pipeline.go).
4. buildReview (reviewplan.go) coerces an APPROVE to COMMENT whenever any reviewer coverage is incomplete.
5. Net: a clean, zero-finding review posts as a comment; on a branch requiring one approval, the PR is unmergeable.

Observed in the wild on two PRs that delete files (a legacy router removal; a Create-React-App → Vite migration): every re-review, including local --rerun, produced outcome: comment with 0 findings across all reviewers, solely because one reviewer skipped a deleted file. A sibling PR with no deletions approved normally.

Fix

Exclude deleted paths (FilePatch.Deleted) from the coverage-completeness decision in buildReviewerCoverage:

• A skipped deletion no longer counts toward incomplete_skipped.
• An unassigned deletion is no longer an incomplete_unassigned gap.
• Skipped files are still reported in full for transparency; only the status decision drops deletions.

The deletion set is derived at the call site from prepared.parsed.Patches and threaded into buildReviewerCoverage.

Tests

• TestBuildReviewerCoverageIgnoresSkippedDeletedFiles — a reviewer skipping a deleted file stays complete, and the skip is still surfaced.
• TestBuildReviewerCoverageDeletedUnassignedFileIsNotAGap — an unassigned deletion doesn't create an unassigned coverage entry.
• Existing buildReviewerCoverage tests updated for the new parameter.

make lint and make test pass. Verified end-to-end by building the patched binary and re-reviewing a real deletion PR: the clean review now posts approve instead of comment.