fix(docker): surface sidecar docker errors + probe for name orphans

Two failure-clarity paper cuts from the cred-proxy debugging:

1. Every docker create / start / network-connect call on the three
   sidecars (pipelock, git-gate, cred-proxy) was piping stderr to
   DEVNULL. A stuck orphan from a previous run produced "failed to
   create pipelock sidecar claude-bottle-pipelock-demo" with no
   pointer at the real cause ("Conflict. The container name ... is
   already in use ..."). Switch each call to capture_output=True and
   include the stripped stderr in the die() message.

2. The agent container had a container_exists() probe in resolve_plan
   that fails fast with a hint, but the sidecars (whose names are
   deterministic from the slug) didn't. So an orphan caused launch()
   to bail deep inside docker create. Add a probe in resolve_plan for
   each sidecar this launch will actually try to create: pipelock
   always; git-gate when bottle.git is non-empty; cred-proxy when
   bottle.cred_proxy.routes is non-empty. Die with a "./cli.py
   cleanup" pointer.

Smoke-tested with an orphaned pipelock-<slug> container — the new
probe fires with the expected hint before any sidecar build/start
work begins.

claude_bottle/backend/docker/prepare.py

@@ -19,9 +19,13 @@ from ...log import die
 from .. import BottleSpec
 from . import util as docker_mod
 from .bottle_plan import DockerBottlePlan
-from .cred_proxy import DockerCredProxy, cred_proxy_url
-from .git_gate import DockerGitGate
-from .pipelock import DockerPipelockProxy
+from .cred_proxy import (
+    DockerCredProxy,
+    cred_proxy_container_name,
+    cred_proxy_url,
+)
+from .git_gate import DockerGitGate, git_gate_container_name
+from .pipelock import DockerPipelockProxy, pipelock_container_name
 
 
 def resolve_plan(
@@ -76,6 +80,29 @@ def resolve_plan(
                 f"clean up old containers with 'docker rm -f <name>'"
             )
 
+    # Probe sidecar container names for orphans from a previous run.
+    # Sidecar names are deterministic from the slug; an orphan would
+    # surface as a docker-create conflict deep inside launch() with no
+    # actionable hint. Fail fast here with a cleanup pointer instead.
+    # Only probe sidecars this launch will actually try to create:
+    # pipelock always; git-gate when bottle.git is non-empty; cred-proxy
+    # when bottle.cred_proxy.routes is non-empty.
+    sidecar_probes: list[tuple[str, str]] = [
+        ("pipelock", pipelock_container_name(slug)),
+    ]
+    if bottle.git:
+        sidecar_probes.append(("git-gate", git_gate_container_name(slug)))
+    if bottle.cred_proxy.routes:
+        sidecar_probes.append(("cred-proxy", cred_proxy_container_name(slug)))
+    for label, sidecar_name in sidecar_probes:
+        if docker_mod.container_exists(sidecar_name):
+            die(
+                f"{label} sidecar container '{sidecar_name}' already exists. "
+                f"This is an orphan from a previous run; clean it up with "
+                f"'./cli.py cleanup' (or 'docker rm -f {sidecar_name}') and "
+                f"retry."
+            )
+
     env_file = stage_dir / "agent.env"
     prompt_file = stage_dir / "prompt.txt"
     prompt_file.write_text("")