docs(changelog): GST-883 fragment

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

changelog.d/gst-883-review-plan-origin-base.md

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+**A stale local `main` no longer turns a /ship review into a full-depth gate review (GST-883).** `gstack-review-plan` handed the raw base name to `gstack-gate-diff`, which diffed against the local branch. With a local `main` four commits behind `origin/main`, the gate scan read a test edit that was already on `origin/main`, so a plain code diff planned as gate class at full depth. On gstack-evals#123 that cost about 15-20M tokens of readers the change did not need (GEVA-117).
+
+### Fixed
+- **`gstack-gate-diff <branch>`** resolves a branch name to its remote-tracking branch (`origin/main` for `main`, origin first), the way `gstack-review-plan`'s own diff and review dispatch already did (#3780). It uses the local branch only when no remote has the branch, and says so on stderr: `no remote-tracking branch for <base>`, or `<remote>/<base> shares no history with HEAD`. A repository with no remote at all uses the local branch without a note. Branches are read by their full ref names, so a tag named `main` or `origin/main` can no longer stand in for the base and hide committed edits from the scan.
+- **`gstack-review-plan`** passes the merge base its own diff used to `gstack-diff-scope` and `gstack-gate-diff`, so the scope flags, the gate scan and the line counts all read the same commits. Its diff reads `refs/remotes/origin/<base>` and `refs/heads/<base>` by full name too.
+
+### Tests
+- `test/review-plan-origin-base.test.ts`: a fixture whose `origin/main` is one commit ahead of the local `main`, with a test edit on `origin/main` and a branch forked from it. `gstack-review-plan --base main` and `gstack-gate-diff main` plan the same class, depth and gate-tagged count as with a fresh `main` (before the fix: gate, full, `RH-1:1` against code, light, none). A tag named `origin/main` or `main` at HEAD leaves the plan and the scanned range unchanged. Also covers the stderr note for a branch no remote has, an unrelated `origin/main`, and the quiet path with no remote.