fix: ship bubblewrap for DSH's command sandbox, keep agents usable without it (#35)

* fix(example): keep agents usable when DSH's inner sandbox has no backend

DSH confines bash commands itself (bubblewrap, else Landlock) and fails closed
with SANDBOX_UNAVAILABLE when no runner is usable. In this hardened pod -
non-root, capabilities dropped, seccomp RuntimeDefault - bubblewrap cannot
start (it needs CAP_SYS_ADMIN) and the pod exposes no Landlock runner, so the
shipped workspace-write default made every command fail:

  sandbox mode "workspace-write" is requested but no sandbox backend is usable

The DSH base bundle at the pinned ref documents DSH_PERMISSION_MODE as the
explicit deployment override, so set it to danger-full-access in both sandbox
templates. The per-user pod is the isolation boundary in this example (its own
pod, PVC and Service; non-root; all capabilities dropped), so a working agent
is worth more than an inner file policy that cannot be enforced. Note that
danger-full-access also turns approval prompts off.

Trade-off documented rather than hidden: an agent command can now read its own
DSH_HOME (including .credentials.yaml) and write anywhere in the container.
Hosts where a runner works should keep workspace-write or read-only.

- main-image docs: DSH_PERMISSION_MODE row plus a note on the runners and the
  fail-closed behavior
- example README: limits bullet and a troubleshooting row with the in-pod
  backend check and how to restore a confined mode
- architecture: tenant-isolation bullet extended
- skill runbook/troubleshooting: operation guidance; never grant CAP_SYS_ADMIN
  or seccompProfile: Unconfined to "fix" this without an explicit decision

Verified: make check (14 manifests), make lint, YAML assertions on both
templates, and the override expression at the pinned dsh-v0.2.1-rc.5 base
bundle (mode: !!js process.env.DSH_PERMISSION_MODE ?? 'workspace-write').

* fix(image): install bubblewrap for DSH's command sandbox

DSH's Linux sandbox runner chain is bubblewrap, then Landlock, and it fails
closed with SANDBOX_UNAVAILABLE when neither works. The image shipped neither,
so a sandboxed agent had no way to execute confined commands.

Add bubblewrap to the runtime stage. Debian ships it unsetuid (0.12.0), so it
still needs unprivileged user namespaces plus the namespace/mount syscalls: in a
hardened pod (all capabilities dropped, RuntimeDefault seccomp) it usually
cannot create one, and DSH then falls back to Landlock or, failing that, to the
DSH_PERMISSION_MODE=danger-full-access override the templates already set.
Where the host allows an unprivileged user namespace, the first-choice runner
now exists, so the confined modes can be restored.

Docs corrected to match: bubblewrap is no longer "absent"; the blocker is the
unsetuid binary's namespace requirement, not a missing CAP_SYS_ADMIN grant.

Verified: make check, make lint; the package exists in the pinned base
(node:24-trixie-slim, bubblewrap 0.12.0-1~deb13u1); a throwaway container with
the pod's flags (--user 1000 --cap-drop ALL --security-opt no-new-privileges)
still cannot create a namespace, which is why the override stays. No local image
build was run.

* ci(image): build and smoke-test the main image on PRs that touch it

The Dockerfile and the pins in versions.env could only be validated by a merge
to main or by running `make image` by hand; neither gives a PR-time signal.

Add ci-image.yaml: on pull requests that change Dockerfile, .dockerignore or
versions.env (and on manual dispatch) it builds linux/amd64 natively with the
pins sourced from versions.env - no registry login, no push - loads the image
locally and smoke-tests it:

- the packaged CLI runs and reports the pinned version
- the container is the non-root node user (1000) and owns /workspace + DSH_HOME
- the bubblewrap command-sandbox runner is present
- the OCI version label matches versions.env
- a container serves /favicon.svg (the same unauthenticated probe the shipped
  manifests use), with logs filtered so the launch token is never printed

Path-filtered like the other on-demand checks, so it is not a required status
check; the published multi-arch build stays in build-main.yaml and release.yaml.

Docs updated in the same change: AGENTS.md repository map and testing table,
docs/gitflow.md checks section, docs/main-image.md build section.

Verified: actionlint 1.7.12 and yamllint 1.38.0 (CI versions) clean over the
whole repository, plus make check and make lint.

---------

Co-authored-by: SREDevOpsAgents <agents@sredevops.org>

docs/gitflow.md

@@ -49,16 +49,25 @@ branch; the PR is squash-merged, so individual branch commits are never part of
 | `CodeQL` | `codeql.yml` | Static analysis for Python and GitHub Actions. |
 | `dependency-review` | `ci.yaml` | Dependency changes introduced by the PR (job `dependency-review`; runs on pull requests only). |
 
-`python` and `manifests` use native `on.pull_request.paths` filtering, so PRs that
-do not touch the gateway or the manifests skip them. The gateway image build and
-`/healthz` are no longer smoke-tested in a dedicated job: `/healthz` is covered by
-the gateway unit tests, the Dockerfile is linted by `lint` (hadolint), and the
-multi-arch image build is published by `build-gateway.yaml` on merge.
+`python`, `manifests` and `build-amd64` use native `on.pull_request.paths`
+filtering, so PRs that do not touch the gateway, the manifests or the image
+definition skip them. The gateway image build and `/healthz` are no longer
+smoke-tested in a dedicated job: `/healthz` is covered by the gateway unit tests,
+the Dockerfile is linted by `lint` (hadolint), and the multi-arch image build is
+published by `build-gateway.yaml` on merge.
+
+`ci-image.yaml` (`build-amd64`) is the PR-side check for the main image: when
+`Dockerfile`, `.dockerignore` or `versions.env` change it builds `linux/amd64`
+with the pins from `versions.env` — no registry login, no push — and smoke-tests
+the result (`--version`, non-root UID, `/workspace` + `DSH_HOME`, the
+`bubblewrap` runner, the version label, and `/favicon.svg` served from a running
+container). The published multi-arch build still happens only in
+`build-main.yaml` on merge.
 
 Because a skipped path-filtered check is not a passing check, only the always-on
 checks (`lint`, `pr-title`, `CodeQL`, `dependency-review`) should be enforced as
-required status checks; `python` and `manifests` are on-demand checks that skip
-when unrelated to the change.
+required status checks; `python`, `manifests` and `build-amd64` are on-demand
+checks that skip when unrelated to the change.
 
 The image build workflows (`_build-image.yaml`, `build-main.yaml`,
 `build-gateway.yaml`) and `e2e-kind.yaml` are **not** required on every PR: the