fix(desktop): finish POSIX SSH spawn publication

[Wenfengcheng]

Summary

Follow-up to #96084, targeting its salv/96061-spawn-fixes branch rather than main so @SmelterLabs' salvaged commit and authorship remain intact.

This closes three remaining gaps in the same managed-SSH spawn path:

• pass the already-expanded update-mutex path directly to Python instead of shell-quoting it a second time;
• capture only the live backend PID instead of combining the short-lived setsid wrapper PID with the backend PID;
• preserve the old global PID-placeholder semantics by using sed .../g.

Related issue: #96024. This PR deliberately does not duplicate #96084's ownership-variable or stale-reaper changes.

Root Cause

expandRemotePath() returns a complete shell word. withRemoteUpdateMutex() still wrapped that word in shq(), so Python could receive literal quote characters instead of the expanded mutex path.

The detached launch also emitted $! twice: the outer setsid ... & echo $! reported a wrapper that had already exited, while the inner shell reported the live backend. In 20/20 WSL probes, child contained both PID lines. That breaks signal handling and lockfile publication.

Finally, #96084's POSIX replacement used first-match sed. Because lock metadata precedes the final pid field, an earlier __PID__ occurrence could be replaced while leaving pid: "__PID__". /g preserves the previous Bash global-replacement behavior.

Fix

• remove only the second shq(mutexPath);
• remove only the outer & echo $!, retaining the inner backend echo $!, setsid/nohup, redirections, and mutex-FD close;
• add the g flag to PID substitution;
• add structural regressions plus a POSIX full-spawn test that verifies mutex creation, one live PID, matching lockfile PID, global placeholder replacement, and reservation cleanup.

How to Verify

cd apps/desktop
npx vitest run --config vitest.config.ts electron/remote-lifecycle.test.ts
npm run typecheck
npx eslint electron/remote-lifecycle.ts electron/remote-lifecycle.test.ts

The three focused tests failed against parent 542736a891 and pass at 994746f5d2.

Test Plan

• Parent RED: all three A/D/C-global regressions fail on fix(desktop): fresh managed-SSH spawns no longer wedge on dash or double-quoted paths (salvage #96061) #96084 head
• remote-lifecycle.test.ts: 91 passed, 3 platform-skipped
• Desktop TypeScript checks: all three projects passed
• Changed-file ESLint passed
• git diff --check and static secret/debug/conflict scan passed
• WSL2 Ubuntu /bin/sh (dash) full ownership payload passed:
  • one live FULL_PID
  • FULL_LOCK_PID matched it
  • expected serve --isolated argv
  • mutex created at the expanded $HOME path
  • reservation released

Platforms tested: Windows 11/Git Bash and WSL2 Ubuntu/dash. I did not independently repeat #96084's external macOS-to-Ubuntu SSH round trip.

Risk Assessment

Medium-low — this touches process lifecycle and shell construction, but the production diff is three narrow lines. Mutex/flock ownership, descriptor closure, reservation protocol, atomic lock publication, token handling, authentication, timeout, and stale-reaper behavior are unchanged.