fix(engines): stop a signal-killed child reporting success

[clawedassistant26]

The bug

runCmd resolves { ok: true, code: null, signal } when a child dies on a signal — Node puts the signal name in signal and leaves code null. Three places that consume it treat a null code as a clean exit:

• src/upgrade.mjs:128 — const ok = r.ok && (r.code == null || r.code === 0)
• src/skills.mjs:69 — same expression, status "installed"
• src/mcp.mjs:115 — same expression, status "added"

So a child killed by the OOM killer, a timeout wrapper's SIGTERM, or Ctrl-C reads as success. moshcode upgrade prints ✓ up to date and counts it in ✓ upgraded N/N; skill install prints installed; mcp add prints added. All three then exit 0, so a wrapper script cannot tell the difference either.

Reproduced on main before touching anything:

runCmd(SIGKILL) -> {"ok":true,"code":null,"signal":"SIGKILL"}
upgrade.mjs ok-expr  => true
runSkillInstall -> [{"key":"claude","status":"installed","code":null}]
runMcpAdd       -> [{"key":"claude","status":"added","code":null}]

The failure lines are unhelpful too: with code == null and no error, upgrade prints a bare ✗ … upgrade failed and integrations.mjs prints a bare failed, with nothing saying what happened.

The fix

Adds two small helpers next to runCmd in src/engines.mjs:

• ranOk(r) — r.ok && r.code === 0. Success now means "exited 0" and nothing else.
• exitReason(r) — "code 128", "SIGKILL", or the spawn error message.

Used at the three status sites. Results now also carry signal, and the failure lines name it, so a killed child reads failed (SIGKILL) instead of an unexplained failed.

Clean exits and non-zero exits behave exactly as before. Display sites that already handled signals (bin/moshcode.mjs, src/tui.mjs, src/cli.mjs, src/commands.mjs) are untouched.

Tests

3 added: 2 in test/engines.test.mjs (one spawns a real bash -c 'kill -9 $$' and asserts the whole {ok, code, signal} shape, one covers clean exit / bad code / ENOENT), 1 each in test/skills.test.mjs and test/mcp.test.mjs.

Verified fail-before / pass-after by stashing only src/:

• unpatched src: 129 pass / 3 fail
• with the fix: 145 pass / 0 fail (was 141 on main)