[draft] fix(agent_subrun): launch a wasm engine through moonrun

[bobzhang]

Follow-up to #1290. With openseek 0.3.1 on mooncakes, moonx bobzhang/openseek/cmd/openseek run '...' works — but only for flat runs. Anything that spawns a child engine died at spawn:

explore produced no report (Failed("subrun spawn failed:
  OSError(\"@process.spawn(): Permission denied\")"))

Cause

self_executable() (cmd/openseek/subrun.mbt) resolves the child from argv[0]. On a wasm build that is the .wasm module itself, which no OS will exec.

Affected: the model-callable explore, review, and subtask tools, and the --review-gate audit — every path that reaches @agent_subrun.run_subrun. The parent turn itself worked fine, which is why it was invisible to CI: the wasm row never spawned a child.

Not affected: --concurrency best-of-N (its attempts are in-process tasks on the same task group, not child engines) and the openseek review subcommand (in-process, no spawn).

Fix

The module needs its runtime in front of it — moonrun <module> -- <args...>, the same shape moonx used to start the parent. The rewrite goes in run_subrun, once, right after the argv is fully assembled (after the --session append, so the fence lands in the right place).

Doing it at that choke point fixes all four callers without touching a signature. self_exe stays a String, agent_explore's public definition is unchanged, and there is no .mbti drift — so openseek_tui and any other consumer of the just-published 0.3.1 are unaffected.

On naming the runtime rather than discovering it

Worth being explicit, since sniffing a file extension is not usually the right instinct.

A host normally hands a program a way back to itself — process.execPath (Node), sys.executable (Python), RbConfig.ruby (Ruby) — and self-respawn is supposed to go through that API. wasm has no equivalent: a module cannot ask who is hosting it. There is also nothing to choose between here, because moonrun is the only host that supplies the FFI this binary needs (process spawn, sockets, filesystem) — a bare WASI runtime could not have run the parent either, so it can never be the answer.

The suffix test reads the child path, not the parent's own backend. That keeps a native parent pointed at a wasm engine working, and leaves the scripted test children (sh, absolute paths, bare names) untouched.

Verification

Before, on the wasm build — three failed delegations. After:

{"event":"subrun_started","id":"sr-1","kind":"explore",...}
{"event":"subrun_finished","id":"sr-1","status":"captured","steps":16,...}
{"event":"tool_result","tool_name":"explore","is_error":false,...2 citations...}

• 5 new whitebox tests over launch_argv cover the rewrite, the empty-argv fence, native passthrough, scripted/PATH children, and .wasm as suffix-only (/build/wasm/openseek and openseek.wasm.bak must not trigger).
• moon test -p bobzhang/openseek/agent_subrun — 33/33 on both native and wasm.
• moon check, moon check --target wasm, moon fmt --check all clean.

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https://claude.ai/code/session_01Xp3xk2xvGLmknC4eCFW3yX