fix: Fix zombie process spawning from shell PATH resolution

[charlesvien]

Problem

fixPath() using execSync with -ilc causes thousands of zombie processes on macOS. The 5s timeout only kills the parent shell, orphaning all children.

Closes #1399

Changes

1. Switch from execSync -ilc to spawnSync -lc to skip .zshrc plugin loading
2. Add killSignal: "SIGKILL" to kill the entire process group on timeout
3. Set POSTHOG_CODE_RESOLVING_ENVIRONMENT env var so users can fast-exit their shell config
4. Cache resolved PATH to shell-env-cache.json with 1-hour TTL to skip shell spawn on subsequent launches

How did you test this?

Manually

[charlesvien]

• fix: Fix zombie process spawning from shell PATH resolution #1435 👈 (View in Graphite)
• main

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