changelog: GSTA-203 PR 5 fragment

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

changelog.d/capy-blobless-clones-on-the.md

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+**The PR and train path check out blobless clones, and the history reads they gate on fail loudly instead of reading a failed fetch as "absent" (GSTA-203 PR 5).** Free Tests' plan job, the Version Gate and E2E Evals' route, screen summary, planner, slices, reruns, bisect and report checked out every file of every commit (about 232 MB) to read a handful of old files. They now check out with `filter: blob:none` (about 17 MB), and git fetches an old file's contents the first time something reads it.
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+### Changed
+- **Blobless checkouts** (`filter: blob:none`) in `free-tests.yml` (free-plan), `version-gate.yml` and `evals.yml` (route, screen-summary, plan-slices, eval-slices, eval-rerun, train-bisect, slices-report). The release, CSO, periodic, marathon, sweep and recorder workflows and the main backstop keep full clones. `test/ci-history-read.test.ts` names each with its reason and fails on any other full-history checkout without the filter, printing the file and line to fix.
+- **History reads fail loudly.** The train-policy reads in the route and backstop jobs and the Version Gate's base `VERSION` read go through `.github/scripts/history-read.sh`. The Version Gate's train classification goes through its TypeScript twin, `lib/history-read.ts`. Each one checks existence with `git ls-tree`, reads with `git show` (3 tries in a partial clone) and fails with `HISTORY_READ_FAILED` naming the ref, the path and the git version (docs/troubleshooting.md#history-read-failed). A file that is really absent keeps its old meaning. A ship policy that is not valid JSON now fails the route and backstop jobs instead of reading as "no train". The Version Gate goes red when `git fetch origin <base>` fails 3 times instead of carrying on.
+- **The eval image asserts partial clone support.** `.github/docker/Dockerfile.ci` makes a blobless clone and fetches a missing blob at build time, so an image whose git cannot do that fails to build.