refactor(docker): drop legacy per-sidecar container_name functions

Same line of cleanup as the supervise rename: the per-sidecar
container names (`claude-bottle-pipelock-<slug>`,
`claude-bottle-egress-<slug>`, `claude-bottle-git-gate-<slug>`)
were docker-network aliases pointing at the bundle, kept so legacy
URLs would keep resolving. Replaces them with short hostnames
(`pipelock`, `egress`, `git-gate`) matching the existing
`EGRESS_HOSTNAME` pattern, and inlines the bundle-loopback URL
(`http://127.0.0.1:8888`) for the in-bundle egress→pipelock hop —
matching what smolmachines already does.

Drops the three `*_container_name` functions, `pipelock_proxy_url`,
and `git_gate_host`. Their callers move to the new constants:
- `PIPELOCK_HOSTNAME = "pipelock"` (claude_bottle/pipelock.py)
- `GIT_GATE_HOSTNAME = "git-gate"` (claude_bottle/git_gate.py)
- `BUNDLE_LOCAL_PIPELOCK_URL` (backend/docker/pipelock.py)

The agent's HTTP_PROXY now reads `http://pipelock:8888` (vs the
old `http://claude-bottle-pipelock-<slug>:8888`); the gitconfig
insteadOf rewrites become `git://git-gate/<repo>.git`. The prepare-
time orphan probe is collapsed onto the bundle container name
(`claude-bottle-sidecars-<slug>`) instead of the four legacy
per-sidecar names that no backend creates anymore.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/docker/launch.py

@@ -65,7 +65,7 @@ from .compose import (
 )
 from .egress import egress_tls_init
 from .pipelock import (
-    pipelock_proxy_url,
+    BUNDLE_LOCAL_PIPELOCK_URL,
     pipelock_tls_init,
 )
 
@@ -149,7 +149,7 @@ def launch(
                 mitmproxy_ca_host_path=egress_ca_host,
                 mitmproxy_ca_cert_only_host_path=egress_ca_cert_only,
                 pipelock_ca_host_path=ca_cert_host,
-                pipelock_proxy_url=pipelock_proxy_url(plan.slug),
+                pipelock_proxy_url=BUNDLE_LOCAL_PIPELOCK_URL,
             )
         supervise_plan = plan.supervise_plan
         if supervise_plan is not None: