feat(backends): add optional pi_embedded backend and config wiring

config/default.yaml

@@ -200,7 +200,7 @@ models:
 # - set `default` to route normal text turns to a backend by default.
 # - per-agent override is available via `agent_configs.<name>.backend`.
 # backends:
-#   default: codex                  # claude_code | opencode | codex | gemini
+#   default: codex                  # claude_code | opencode | codex | gemini | pi_embedded
 #   native:
 #     enabled: true
 #   claude_code:
@@ -223,6 +223,13 @@ models:
 #     # path: /usr/local/bin/gemini
 #     # args: ["-p", "{prompt}"]
 #     # timeout_ms: 120000
+#   pi_embedded:
+#     enabled: false
+#     # timeout_ms: 120000
+#     # no_tools_mode: true         # keep Pi path text-only in canary; force native for tool-like prompts
+#     # model: openclaw-default     # optional model/session selector passed to Pi runtime
+#     # system_prompt_mode: hybrid  # flynn | pi_default | hybrid
+#     # module: "@badlogic/pi-agent-core"   # optional module override
 
 # Optional: Kubernetes / homelab awareness tools (k8s.pods, k8s.deployments, k8s.logs)
 # k8s:
@@ -321,7 +328,7 @@ memory:
 #   assistant:
 #     system_prompt: You are helpful.
 #     model_tier: default
-#     backend: native          # native | codex | claude_code | opencode | gemini
+#     backend: native          # native | codex | claude_code | opencode | gemini | pi_embedded
 #   coder:
 #     system_prompt: Write code.
 #     model_tier: complex