Fix "posix_spawnp failed" by restoring node-pty spawn-helper execute bit

[LonestoneBot]

Problem

Opening a terminal fails with:

Error: Failed to spawn shell: posix_spawnp failed

Root cause

On macOS and Linux, node-pty launches the shell by exec'ing its bundled native
spawn-helper binary. When the plugin is distributed as an unpacked bundle
(rather than a fresh npm install), the archiving/extraction step can drop the
execute bit on that binary:

-rw-r--r--  spawn-helper   # should be -rwxr-xr-x

Without +x, posix_spawnp returns EACCES, which node-pty surfaces as
posix_spawnp failed. Diagnosis on an affected install (Intel macOS,
node-pty 1.1.0): the binary is a valid Mach-O x86_64 executable with no
Gatekeeper quarantine — only the execute bit is missing. chmod +x on the
helper immediately fixes it.

Fix

Add ensureSpawnHelperExecutable(), run once at startup right after node-pty
is loaded. It locates the helper in both the modern prebuilds/<platform-arch>/
layout and the legacy build/Release/ layout, and restores the execute bit when
it's missing.

• Best-effort: failures are logged, never thrown.
• No-op on Windows (which uses ConPTY, no spawn-helper) and no-op when the bit
  is already set — so it can't regress a healthy install.
• Matches the file's existing defensive style (findModule,
  prioritizeUserNpmGlobalBin).

Testing

On an affected install:

$ chmod -x .../node-pty/prebuilds/darwin-x64/spawn-helper   # reproduce
$ node dist/server.js
[web-terminal] restored missing execute bit on .../prebuilds/darwin-x64/spawn-helper
{"ready":true,"port":64034}

The helper is restored to -rwxr-xr-x and terminals spawn normally.
tsc build passes. Only src/server.ts changes (dist/ is gitignored).

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