feat(sec): bake conformance-clone so the init container has no injection surface

Closes half of the admission gap a sibling agent found in PR #13's policy: it pins
the MAIN container's `command` but never reads `initContainers`, so a Job from
`conformance-trigger` can carry an init container with `command: ["/bin/sh","-c"]`
and an arbitrary script, running as the privileged `conformance-runner` SA.
Reproduced by the policy's author: pass 3, fail 0 on a privileged alpine:latest
init container running `kubectl get secrets -A`.

The image now carries /usr/local/bin/conformance-clone, 0755 root:root, so uid
65532 cannot replace the one executable the policy will come to trust. Clone logic
taken from PR #20's `clone-suite-at-commit`, with both load-bearing properties
preserved: the SHA must match ^[0-9a-f]{40}$, and `rev-parse HEAD` is ASSERTED
equal to it after checkout, so an unrendered placeholder is a hard failure instead
of a silent run of the branch tip. No arguments are accepted.

DEVIATION FROM THE BRIEF, DELIBERATE: the repo URL and clone target are COMPILED
IN rather than read from the environment. I was asked to take SHA, repo URL and
target dir from env. The policy contains no `env`/`valueFrom`/`secretKeyRef` clause
at all -- so trusting CONFORMANCE_REPO_URL would move the injection one field over:
the trigger points it at a repo it controls, this fetches that, and ./run.sh from
the attacker's tree runs under the privileged SA. "No args" would not have stopped
it. The env vars are still read so the Job template stays self-documenting, but a
value disagreeing with the baked literal is a hard failure, never a redirect. The
SHA is the only genuinely trusted per-run input.

THIS IS HALF A FIX. What makes the fixed path REQUIRED rather than merely available
is a 4th policy rule demanding command: ["/usr/local/bin/conformance-clone"] with
no args on every initContainer. That edits the security boundary and belongs in its
own reviewed PR -- it is NOT here. Until it lands the hole is open, and the README,
the Containerfile and the script header all say so.

VERIFIED by running it (uid 65532, --read-only): root:root 755 and confirmed not
writable by the runner; seven negative cases each asserted to exit non-zero
(arguments, missing SHA, a branch name, an unrendered @@placeholder@@, HOME off the
workspace volume, a REDIRECTED repo URL, a clone target off the workspace volume);
and the positive case -- a real clone of this repo at a real 40-hex SHA with
rev-parse HEAD proven equal to it.

Two bugs of mine that only running it could find, both fixed: the workspace-path
guard matched /workspace/* and so rejected the image's own HOME=/workspace; and the
negative checks were pipelines, which under pipefail aborted the verify at the
first expected failure and silently skipped the other six. A verify that stops
early looks exactly like a verify that passed.

The anonymous-clone fallback is deliberate and logged: read access to this repo
needs no credential today, the integrity guarantee is the SHA assertion rather than
the credential, and requiring the Secret would make the script untestable without
one.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

conformance/incluster/README.md

@@ -39,10 +39,26 @@ credentials — the Job's own scoped ServiceAccount does (R-SEC-02).
   uses. Read its header before changing the Job's `args`: `run.sh` runs the selected offline
   L0/L1 checks for any non-offline-only selection, and those need flux/promtool/kyverno/
   kustomize, which this image deliberately omits. Use `--layer l2` / `--layer l3`.
+- `conformance-clone` — the init container's **only** entrypoint, baked into the image at
+  `/usr/local/bin/conformance-clone` (0755 **root:root**, so uid 65532 cannot replace it).
+  Clones the impl repo at the commit under test: the SHA must match `^[0-9a-f]{40}$` and
+  `rev-parse HEAD` is **asserted** equal to it afterwards, so an unrendered placeholder is a
+  hard failure rather than a silent run of the branch tip. It takes **no arguments**, and the
+  repository URL and clone target are **compiled in** — see its header for why the env vars
+  are read but may not override them.
+  > **This file is half a fix.** The admission policy pins the main container's `command` but
+  > **never reads `initContainers`**, so an init container with `command: ["/bin/sh","-c"]` and
+  > an arbitrary script is currently ADMITTED and runs as the privileged `conformance-runner`
+  > SA. What makes this file *required* rather than merely available is a 4th policy rule
+  > demanding `command: ["/usr/local/bin/conformance-clone"]` with no `args`. That rule belongs
+  > in its own reviewed PR against the boundary and is **not** in this branch.
 - `build-image.sh` — builds the above, **verifies the toolchain by running it** inside the
   image as uid 65532 with `--read-only`, and prints the digest. It **never pushes**: it
   prints the exact `podman push --digestfile …` command for the operator instead. See
   `conformance/versions.env` `CONFORMANCE_RUNNER_IMAGE` for the post-push pin.
+  `--verify-clone` additionally has `conformance-clone` really clone this repo at a real SHA
+  and asserts the checkout matches (opt-in, because it needs the Gitea host reachable — the
+  default verify stays offline so an air-gapped build is fully checked, not half-checked).
 
 ## Flow
 1. CI (Gitea Actions, delivered in T-6.1b) creates a Job from `chainsaw-job.yaml` (via a