docs(fleet): record durable-launch findings + runtime-default policy

Correct the launch-path finding (PATH, not TTY), record the validated durable
real-agent recipe (pi on openai-codex/gpt-5.5), the Codex-default/Claude-reserved
policy, and the fleet-init boot-survival automation TODO.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RMoEx7hfdFGjUiCHuN1RRi

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)