feat(msb): derive default sandbox-template image from agent token

[mogul]

Context

AI-assisted change.

When acq provisions through the msb backend and the user does not set an image, msb always used docker.io/docker/sandbox-templates:shell-docker. That differs from sbx, where sbx run opencode . uses docker.io/docker/sandbox-templates:opencode-docker — which bakes in the agent binary and significantly shortens startup. This PR brings the msb path to parity.

Fixes #404. Also does the enabling cleanup from #377 (single-sourcing the agent list). prime-agent is not added here.

What changed

• Agent-derived default image (msb): when no explicit image override is set, msb derives docker.io/docker/sandbox-templates:<agent>-docker for known agents (shell → shell-docker), matching sbx's template naming. Precedence: ACQ_MSB_IMAGE > --image/ACQ_IMAGE > agent-derived default > shell-docker fallback.
• Fallback only on not-found: on msb create failure, msb retries once with shell-docker only when the image was agent-derived, the error is registry not-found, and no sandbox was created. Auth/network failures and explicit image overrides never fall back silently. The not-found classifier was live-verified against msb (Docker Hub + ghcr.io both return manifest unknown).
• Single-source agent catalog (Single-source the agent list (KNOWN_AGENTS dup + 4 case statements + 3 docs) #377): new acq.backends/agents.sh holds ACQ_KNOWN_AGENTS, acq_is_known_agent, acq_agent_template_image, and the msb install-recipe check. Both adapters source it; the duplicated KNOWN_AGENTS definitions and parallel is_known_agent case statements are removed. New test/bats/36-agent-catalog.bats enforces catalog/adapter parity and that the list lives only in the shared file.
• Docs/ADRs: BACKEND_GUIDE.md, ADR-0022, ADR-0011 updated for the derived default + fallback; howto/msb.md and howto/sbx.md agent lists corrected and pointed at agents.sh.

Verification

scripts/test-acq-bats (full)                              376 ok, 0 not ok
scripts/test-acq-bats test/bats/35-image-override.bats     22 ok, 0 not ok
scripts/test-acq-bats test/bats/36-agent-catalog.bats       5 ok, 0 not ok
classifier probe vs real msb stderr    manifest unknown -> fallback; unauthorized -> no fallback
shellcheck --severity=warning (acq + backends + tests)    clean
markdownlint (touched docs)                               0 issues

Live msb probe (host with a sandbox-capable runtime), confirming the classifier wording:

$ msb create ... docker.io/docker/sandbox-templates:definitely-not-a-real-tag-xyz
error: image error: registry error: ... [OCI API error: manifest unknown]   rc=1
$ msb create ... ghcr.io/gsa-tts/definitely-private-nonexistent:v1
error: image error: registry error: ... [OCI API error: manifest unknown]   rc=1

Rollback

Revert this commit; no state migration. Behavior returns to always using shell-docker on msb.

Security impact

None. No change to auth, secrets, or egress. The image name is derived from a charset-validated known-agent token; unknown/unsafe tokens fall back to shell-docker. Explicit-image failures are never masked.