release: move the version pins to 0.1.5, now that the tag and its images exist

[rahulv8]

The second half of the v0.1.5 release, split out because three guards read the
tag and the tag is cut from the release PR's merge commit. A chart pinned to
0.1.5 before 0.1.5 was published is a chart that promises images nobody can
pull, so these lines could only move once #66 had merged and
v0.1.5 existed.

What moves

• chart version and appVersion → 0.1.5
• install-k8s.sh RSYNC_CHART_VERSION → 0.1.5, which
  test_an_oci_chart_is_pinned_to_the_release_version requires to equal appVersion
• the helm install --version / helm pull --version lines and the pinned
  values-gke.yaml URL, in README.md, the chart README and docs/deployment/kubernetes.md
• ## [Unreleased] → ## [0.1.5] - 2026-09-23

The dated claims are re-measured, not renumbered

Chart.yaml and kubernetes.md each assert that every image the chart can name
answers an anonymous pull, multi-arch, as of a date; the chart README says the
same of latest. Bumping the version in those sentences without re-running the
check would turn a measurement into a copied number. Today's results, which is
what they now carry:

Check	Result
Publish run 35822010242	36/36 jobs, 0 failed
Anonymous manifest fetch, all 34 packages (13 services + 21 connectors) at 0.1.5 and latest	34 probed, 0 failures, every one an OCI index listing linux/amd64 + linux/arm64
helm pull oci://ghcr.io/rsync-ai/charts/rsync-ai --version 0.1.5	pulled, sha256:fdb2db2c…; published version/appVersion both 0.1.5
helm template of the pulled chart	22 image references, all at tag 0.1.5, across 12 images — all within the 34

The probe was armed before being trusted: pointed at a ref that does not exist
it reports MISSING and exits non-zero, and helm pull --version 9.9.9 fails
with not found.

deploy/helm/rsync-ai/Chart.yaml on main said 0.1.4 throughout the release
and that was correct, not a bug — docker-publish.yml's publish-chart job
derives both the chart version and appVersion from the git tag, so the
published artifact was 0.1.5 regardless. This PR syncs the checkout to what
shipped; it is not repairing a broken chart. The file's own comment says as much,
and only matters for helm install ./deploy/helm/rsync-ai from a checkout.

Verification

Run in a throwaway clone rather than in the tree being pushed:

• 21 doc guards (the list taken from doc-links.yml, not assumed): 170 passed, 18 skipped
• test_install_k8s_defaults_work.py, test_documented_install_commands_render.py,
  test_shipped_images_are_anonymously_pullable.py, test_shipped_images_are_pinned.py:
  208 passed, 4 skipped
• test_changelog_versions_name_real_tags.py: 4 passed, none skipped — and with the
  heading rewritten to ## [9.9.9] it fails, so the green is load-bearing rather than
  a tagless skip

Until this merges, a Kubernetes install has to pass RSYNC_CHART_VERSION=0.1.5
explicitly, which is what the release notes say.

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