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/firecracker/consolidated_launch.py

@@ -50,6 +50,7 @@ class LaunchContext:
     source_ip: str          # the VM's guest IP — the attribution key
     gateway_ca_pem: str     # the shared gateway CA the provisioner installs
     orchestrator_url: str
+    env_var_secret: str = ""  # encryption key injected into the agent's env
 
 
 def launch_consolidated(
@@ -80,6 +81,7 @@ def launch_consolidated(
         source_ip=guest_ip,
         gateway_ca_pem=infra.gateway_ca_pem(),
         orchestrator_url=url,
+        env_var_secret=reg.env_var_secret,
     )