fix(fleet): preflight pane runtimes before install (#1256)

docs/fleet/FLEET-LAUNCH.md

@@ -59,6 +59,28 @@ valid allowed local data can move to `.env.local`; invalid legacy input is priva
 Diagnostics expose only rule code, key name, and a SHA-256 content hash. They do not reveal command
 text, credentials, or other values.
 
+## Pane executable preflight
+
+The fleet install, install-systemd, and doctor commands plus the session launcher use
+**pane-runtime-path.sh** as the single pane-PATH implementation. Install inspects every distinct
+roster runtime and effective MOSAIC_RUNTIME_BIN pair before creating holder identity, tool,
+projection, or unit files. Doctor reports the same checks as JSON.
+
+A resolved command is not automatically executable. The helper reads a script shebang, unwraps the
+common “/usr/bin/env node” and “/usr/bin/env -S node …” forms, then resolves the declared command
+against the pane PATH. When Node is the declared interpreter, the helper runs the side-effect-free
+“node --version” probe. It does not run “mosaic --version”, whose startup update check can write cache
+state. Native binaries have no PATH-resolved shebang dependency and retain their normal executable
+check. Failures name the executable or runtime, requesting roster rows, searched pane PATH,
+dependency, and runtime install command.
+
+Supported runtime install commands are:
+
+- **Claude:** curl -fsSL https://claude.ai/install.sh | bash
+- **Codex:** npm install -g @openai/codex
+- **OpenCode:** npm install -g opencode-ai
+- **Pi:** npm install -g @earendil-works/pi-coding-agent
+
 ## Launch and stop behavior
 
 The launcher obtains the agent's socket only from the validated generated projection. It creates or