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/egress.py

@@ -32,14 +32,6 @@ EGRESS_PIPELOCK_CA_IN_CONTAINER = (
 )
 
 
-def egress_container_name(slug: str) -> str:
-    """The legacy per-sidecar container name. Kept as a function so
-    the renderer can register it as a docker-network alias on the
-    bundle — any code still referring to `claude-bottle-egress-<slug>`
-    resolves to the bundle's IP."""
-    return f"claude-bottle-egress-{slug}"
-
-
 def egress_tls_init(stage_dir: Path) -> tuple[Path, Path]:
     """Mint the per-bottle egress MITM CA via host `openssl req`.