feat(egress): inject per-session canary token into sidecar and agent environments

EgressPlan gains a `canary: str` field (default "") populated in Egress.prepare()
using secrets.token_urlsafe(32).  Each launched bottle:

  - sidecar receives EGRESS_TOKEN_CANARY=<value> (literal env entry, scanned by
    existing known-secrets detector without any detector code changes)
  - agent receives BOT_BOTTLE_CANARY=<value> (visible fake secret that signals
    exfiltration with zero false positives if it appears in outbound traffic)

Docker compose and macos-container backends updated; smolmachines shares docker
compose and so picks this up automatically.  Unit tests cover canary uniqueness,
detection via scan_known_secrets, and EgressPlan backward-compat default.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/egress.py

@@ -10,6 +10,7 @@ specific and lives on concrete subclasses (see
 from __future__ import annotations
 
 import dataclasses
+import secrets
 from abc import ABC
 from dataclasses import dataclass
 from pathlib import Path
@@ -65,6 +66,7 @@ class EgressPlan:
     mitmproxy_ca_host_path: Path = Path()
     mitmproxy_ca_cert_only_host_path: Path = Path()
     log: int = 0
+    canary: str = ""
 
 
 def egress_manifest_routes(
@@ -324,12 +326,17 @@ class Egress(ABC):
         routes_path = stage_dir / EGRESS_ROUTES_FILENAME
         routes_path.write_text(egress_render_routes(routes, log=log))
         routes_path.chmod(0o600)
+        # Generate a per-session canary token.  The sidecar receives it as
+        # EGRESS_TOKEN_CANARY (scanned by the existing known-secrets detector);
+        # the agent receives it as BOT_BOTTLE_CANARY (a visible fake secret).
+        canary = secrets.token_urlsafe(32)
         return EgressPlan(
             slug=slug,
             routes_path=routes_path,
             routes=routes,
             token_env_map=egress_token_env_map(routes),
             log=log,
+            canary=canary,
         )
 
 __all__ = [