add sandbox runtime control endpoints

docs/009-sandbox-http-control-and-runtime-params.md

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+# 009 Sandbox HTTP control and runtime params
+
+## Context
+- Sandbox API must support explicit delete and richer create params
+- Clients need an internal Docker-network endpoint for agent traffic
+- MVP accepts trusted internal callers and does not enforce auth yet
+
+## Decision
+- `POST /api/v1/create` accepts `chat_id`, `agent_id`, and absolute `volume_host_path`
+- `AGENT_ID` is passed to the sandbox container environment
+- The request volume is bind-mounted read-write at configured `volume_mount_path`
+- Sandbox containers join configured Docker network `network_name`
+- Create returns endpoint `ip:agent_service_port` from that Docker network
+- Reuse is allowed only when `agent_id` and `volume_host_path` match
+- Mismatch returns sandbox conflict without starting a new container
+- `DELETE /api/v1/sandboxes/{chat_id}` deletes the active sandbox without auth
+
+## Consequences
+- Absolute host path is accepted as an MVP risk
+- External clients must run in or join the configured Docker network