fix(fleet): durable runtime PATH for detached agent launch (#581)

docs/fleet/north-star.md

@@ -115,6 +115,11 @@ Every artifact, starting Phase 2, MUST:
 - Observation: **read-only default, opt-in takeover**.
 - Multi-host: **designed-for from day one**; control plane **rides federation (W1)**.
 - Delivery: **CLI-first now**, dogfood against the live stub fleet; webUI deferred to Phase 5.
+- Runtimes: fleet agents default to **Codex / pi-on-Codex**; **Claude is reserved for Claude
+  Code only** (avoid alternate-harness API pricing). Validated durable recipe:
+  `mosaic yolo pi --model openai-codex/gpt-5.5:high`. Durable detached launch requires the
+  runtime-bin on PATH (baked into the pane command) + boot-survival (`enable` + linger),
+  which `fleet init` should automate.
 
 ## Assumptions (veto-able)