docs: document jail root_path + staged env payloads

Match the docs to the shipped staged-env code: add the JailConfig root_path
field, a 'Staged env payloads' section (prepare_spawn_command writes env.sh/
launch.sh under /var/run/colibri-stage/<id>/), resolve the mdo-env-passthrough
open item, and add root_path to the external-MCP example.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/COLIBRI-EXTERNAL-MCP-PROTOTYPE.md

@@ -43,7 +43,7 @@ Schema:
       "env": {
         "DEMO_MODE": "1"
       },
-      "jail": { "name": "mcp0" }
+      "jail": { "name": "mcp0", "root_path": "/usr/local/bastille/jails/mcp0/root" }
     }
   }
 }

docs/COLIBRI-JAILED-AGENT-SPAWN-DESIGN.md

@@ -24,6 +24,9 @@ as before. The field that is set picks the jail lifecycle:
 - **`path`** — create an **ephemeral** jail with `jail -c … command=<binary>`,
   which exists only while the agent runs and is removed when it exits (no teardown
   needed).
+- **`root_path`** — host-visible root path for a named jail; required when staged
+  env/working-dir payload delivery is needed. Falls back to `path` for ephemeral
+  jails.
 - optional `ip4` (`inherit` by default) and `user` (in-jail user, `jexec` path).
 
 `jail_wrap()` turns `(binary, args)` into the `(program, argv)` to exec. stdio is
@@ -53,19 +56,31 @@ just `jexec`. We choose the escalation per host via `PrivMode`
   keep the daemon unprivileged.
 - **`none`** — run the jail command directly (already root, or tests).
 
+## Staged env payloads
+
+When a jailed spawn needs env vars or a working dir, `prepare_spawn_command()`
+writes a 0600 `env.sh` (sorted, single-quoted exports) and a `launch.sh` wrapper
+into a staged directory under the jail's `root_path` at
+`/var/run/colibri-stage/<id>/`. The jail command runs `/bin/sh launch.sh`, which
+sources the env file and `cd`s to the working dir before `exec`-ing the agent
+binary. This bypasses the env-passthrough problem entirely — no reliance on
+`jexec`/`mdo` inheriting env vars.
+
+The staged directory is cleaned up when the agent stops, fails, exits early, or
+encounters a poll error. The same mechanism is used by the external-MCP host for
+jailed MCP servers.
+
 ## Open items
 
 - **Teardown:** ephemeral `jail -c command=` self-cleans; reaping a deeply nested
   in-jail process tree may want a process-group kill (follow-up).
-- **mdo env passthrough:** verify on FreeBSD that `mdo` propagates the injected
-  `COLIBRI_*` / provider env into the jailed process; if not, pass via `jexec`
-  args or a 0600 env file.
 - **Jail filesystem provisioning** (ISO / deploy): the jailed binary needs its
   runtime + work dir — a pre-provisioned persistent jail, or nullfs mounts for an
   ephemeral one.
 
 ## References
 
-- `crates/colibri-daemon/src/spawner.rs` — `JailConfig`, `PrivMode`, `jail_wrap`
+- `crates/colibri-daemon/src/spawner.rs` — `JailConfig`, `PrivMode`, `jail_wrap`,
+  `prepare_spawn_command`, `PreparedSpawnCommand`
 - `crates/colibri-daemon/src/lib.rs` + `socket.rs` — `jail` on the spawn-agent command
 - `crates/colibri-mcp/src/external.rs` — jailed external MCP servers