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`mosaic fleet start` returned 0 over three dead panes. The launcher knew, and said the wrong thing at the wrong severity to the wrong layer. The pane runs `mosaic yolo <runtime>` under PANE_PATH with a cleared environment. When that binary is absent the pane dies in under a second, tmux destroys the session, and the diagnostic goes with it. The launcher then found no PANE_PID, printed a WARNING about the *heartbeat sidecar*, and exited 0 — so systemd logged "Finished ... successfully" and `fleet start` reported success. `fleet ps` was the only component telling the truth. Two changes, both in start-agent-session.sh: 1. Before any effect, resolve `mosaic` and the roster's runtime against PANE_PATH — the pane's own view of the path, not the launcher's. `mosaic yolo <runtime>` calls checkRuntime(runtime) and looks for a binary named exactly like the runtime, so this asks the same question the pane will ask a moment later, while an operator can still see the answer. Absent binary -> exit 69, code=missing-binary, no session created. 2. Replace the dead-pane WARNING+exit-0. An absent session one second after new-session is a runtime that died on startup, not a heartbeat problem -> exit 69, code=pane-did-not-survive, with the command to run by hand to see why. A present session with no pane PID after five attempts -> code=pane-pid-unresolved. Neither branch kills the session; destroying a possibly-live pane on a guess is worse than leaving it for inspection. Exit 69 (EX_UNAVAILABLE) is deliberate: the 64s already in this file mean the projection was bad, and here the data is fine and the host is not ready. Callers separate the cases by `code=`, the same way fail_env's codes share 64. This propagates for free. `fleet start` calls runChecked() for the holder and each agent, and runChecked throws on non-zero, so layers 4 and 5 stop lying without a TypeScript change. Two adjacent defects are left for a follow-up issue rather than widened into this diff: the per-agent loop aborts on the first failure instead of attempting all and reporting an aggregate, and runChecked's bare throw surfaces the launcher's message under a Node unhandled-rejection stack trace because program.parse() is synchronous. Tests: - test-start-agent-session.sh gains three cases: `mosaic` absent from the pane path, the runtime absent from the pane path, and a pane that does not survive. Each was verified individually red against the unmodified origin/next launcher. - The two cases asserting a valid launch now supply a pane PID. Until now the suite's one success path was itself a dead pane the launcher reported as fine. - The harness fakes `npm` so PANE_PATH stops depending on whatever the host has installed, and fails loudly if the host provides `mosaic` or `pi` in the system path, where the missing-binary cases would not be measurable at all. - test-fleet-units.sh gains a `pi` shim in its runtime bin. The real-tmux harness named `pi` in its roster and never installed it; the new preflight caught it. Refs #1241