gate: define the build-image pre-push task (daemonless Dockerfile check)

The git-push gate (handler.hooks.gate -> verify.run_build) shells
`mise run build-image`, but .mise.toml never defined that task, so every agent
push was hard-blocked with "no task build-image found". Define it.

The agent sandbox and control image ship no container daemon or builder (by
design — no Docker socket, no --privileged), so a real image build can't run at
push time. Use hadolint as a daemonless soundness check on both Dockerfiles,
with .hadolint.yaml pinning the failure threshold to errors so genuine
Dockerfile mistakes block the push while the deliberate, commented style
choices (unpinned apt/npm/go) don't. The authoritative end-to-end multi-arch
build-and-push stays in CI (.github/workflows/docker*.yml) on every PR to main.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.hadolint.yaml

@@ -0,0 +1,9 @@
+# Config for the daemonless `mise run build-image` pre-push gate (see .mise.toml).
+# Only genuine Dockerfile errors (bad syntax / invalid instructions) should block a push;
+# the deliberate, commented choices in the Dockerfiles are not defects:
+#   DL3008 — apt packages are intentionally unpinned (rolling base image).
+#   DL3016 — `npm install -g @anthropic-ai/claude-code` tracks the latest Claude Code CLI.
+#   DL3062 — `go install ...@latest` is deliberate (forge tracks its latest release).
+#   DL4006 — the piped NodeSource/mise setup runs under the default shell on purpose.
+# The authoritative end-to-end multi-arch build runs in CI (.github/workflows/docker*.yml).
+failure-threshold: error

.mise.toml

@@ -1,7 +1,10 @@
-# Handler dogfoods its own gate: this repo defines the canonical `test` task the
-# control layer's Stop hook enforces. Any project Handler manages carries one of these.
+# Handler dogfoods its own gate: this repo defines the canonical `test` and `build-image`
+# tasks the control layer enforces (the Stop hook runs `test`; the git-push gate runs
+# `test` then `build-image`). Any project Handler manages carries these.
 [tools]
 python = "3.11"
+# Backs the daemonless `build-image` gate below (see that task).
+hadolint = "2.14.0"
 
 [tasks.test]
 description = "Run the test suite"
@@ -14,3 +17,13 @@ run = "ruff check ."
 [tasks.verify]
 description = "Lint then test"
 depends = ["lint", "test"]
+
+# The pre-push gate (handler.hooks.gate -> verify.run_build) shells `mise run build-image`
+# to prove the Dockerfiles are sound before a push. The agent sandbox and control image
+# ship no container daemon/builder (by design — no Docker socket, no --privileged), so this
+# is a daemonless soundness check via hadolint rather than a real image build. The
+# authoritative end-to-end multi-arch build-and-push runs in CI (.github/workflows/docker*.yml)
+# on every PR to main. Failure threshold is pinned to errors in .hadolint.yaml.
+[tasks.build-image]
+description = "Validate the Dockerfiles are sound (daemonless pre-push gate)"
+run = "hadolint Dockerfile Dockerfile.control"