refactor(de-sidecar): purge the "sidecar" name from live code, tests, and current docs

Completes the de-sidecar cleanup: no live code, test, current doc,
script, or nix file mentions or is named "sidecar" any more. Only the
dated PRD/research docs keep the term as historical record (agreed on
the #385 thread).

- Rename `sidecar_init.py`→`gateway_init.py` was done earlier; this pass
  sweeps the remaining descriptive uses: the egress / git-gate / supervise
  components are the gateway's *daemons*, the shared container is the
  *gateway*, the old per-bottle container was the *companion container*.
- Rename `tests/integration/test_sidecar_bundle_image.py`→`test_gateway_image.py`
  and its class; update `docs/ci.md` + `tests/README.md` for the renamed/
  removed integration tests.
- `SIDECAR_PORTS` shell var in `scripts/firecracker-netpool.sh`→`GATEWAY_PORTS`.

Full unit suite green (bar the pre-existing `/bin/sleep`-missing env
errors in test_gateway_init); docker integration — gateway singleton,
broker, real two-bottle multitenant isolation, and the gateway-image
build — all pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

bot_bottle/contrib/claude/agent_provider.py

@@ -4,7 +4,7 @@ The Claude-specific behavior previously inlined under
 `agent_provider.agent_provision_plan` (claude.json trust marker,
 api.anthropic.com egress route, OAuth-token placeholder), plus
 the `claude mcp add` invocation that registers the supervise
-sidecar in claude-code's user config (PRD 0013)."""
+gateway in claude-code's user config (PRD 0013)."""
 
 from __future__ import annotations
 
@@ -293,7 +293,7 @@ class ClaudeAgentProvider(AgentProvider):
         supervise_url: str,
     ) -> None:
         """Run `claude mcp add` inside the agent guest to register the
-        supervise sidecar in claude-code's user config (~/.claude.json).
+        supervise daemon in claude-code's user config (~/.claude.json).
 
         Failure is logged but not fatal — the bottle still works without
         the entry; the operator can register it manually."""