feat(secrets): encrypt egress tokens at rest with per-bottle ENV_VAR_SECRET

Implements the interim secret-provider design (PRD prd-new-secret-provider):
each agent receives a random ENV_VAR_SECRET injected into its container env
at launch. The host uses this key to encrypt each egress auth token value
(HMAC-SHA256 CTR mode, stdlib-only) and store it in a new
bottled_agent_secrets table (one row per env var, key column plaintext for
auditing). The key never touches the DB.

On infra container restart the in-memory token map is lost. launch_consolidated
now calls _reprovision_running_bottles after ensure_running: for each
registered bottle still alive on the gateway network it execs
`printenv ENV_VAR_SECRET` into the agent container and posts the result to the
new POST /bottles/<id>/reprovision_gateway control-plane endpoint, which
decrypts the stored rows and restores _tokens — no manual intervention needed.

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

bot_bottle/backend/docker/bottle_plan.py

@@ -39,6 +39,10 @@ class DockerBottlePlan(BottlePlan):
     # (egress proxy credentials, git-gate/supervise headers); set by launch
     # from the orchestrator registration. Empty pre-registration.
     identity_token: str = ""
+    # Encryption key for the agent's stored egress secrets; injected into the
+    # agent container as ENV_VAR_SECRET via the compose subprocess env (bare
+    # name — value never written to the compose file). Empty pre-registration.
+    env_var_secret: str = ""
 
     @property
     def container_name(self) -> str: