docs: CHANGELOG fragment for GST-969

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

changelog.d/gst-969-qa-tree-writes.md

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+### Fixed
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+- **/ship's exploratory QA no longer writes reports into your project (GST-969).** Step 9.2.1 used to put its QA reports, deadline file and captured evidence in `.gstack/qa-reports/` inside the repository under review. In a project that doesn't ignore `.gstack/`, those files showed up as untracked changes, so they could land in the PR and change the tree the review certified. The new `gstack-qa-evidence report-dir` command creates a fresh run directory under `.gstack/tmp/qa-reports/` and adds `/.gstack/tmp/` to the repository's git exclude file, and /ship's QA step now uses it. It refuses a symlinked `.gstack`, a `.gitignore` rule that un-ignores `.gstack/tmp`, and a directory outside a git repository. /review's Step 4.7 still uses `.gstack/qa-reports`: /review's prompt is already at its size cap.