fix(pr-review-canvas): preserve header-like source lines inside hunks

[rudycelekli]

A JavaScript change from -- count; to ++ count; produces unified diff lines --- count; and +++ count;. The renderer currently mistakes these for file headers and silently drops both substantive changes from the review.

Track whether parsing has entered a hunk. Skip ---/+++ file headers only outside hunks, preserving source lines with those prefixes inside a hunk. A subsequent diff header resets that state for full multi-file diffs.

Validation: node --test cursor-team-kit/skills/pr-review-canvas/tests/header-content.test.cjs passes 3 tests using real Git-generated patches. Both API-patch and full-diff regressions fail on the unchanged base; the ordinary replacement control passes. JavaScript syntax and git diff --check pass.

Existing #482 and #489 fix import filtering/whitespace transformations; inspection of their current renderer patches confirms both retain the unconditional header-prefix skip addressed here.

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Note

Low Risk
Localized diff-parsing fix in the review canvas renderer with targeted regression tests; no auth, data, or API surface changes.

Overview
Fixes a bug where pr-review-canvas dropped real code changes when deleted/added lines looked like diff file headers (e.g. -- count; → --- count; and ++ count; → +++ count;).

The diff parser now tracks inHunk: it still skips --- / +++ only before the first @@, but treats those prefixes as normal add/delete lines inside hunks. A new diff line resets hunk state for multi-file full diffs.

Adds header-content.test.cjs with Git-generated patches for API-style hunks and full diffs, plus a simple old/new control.

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