#22721: fix(exec): keep child runs attached on macOS for reliable output

## Summary Fixes #22350. On macOS LaunchAgent sessions, spawning exec child processes with `detached: true` can cause stdout/stderr pipes to behave like fire-and-forget runs (command exits quickly with no captured output). This made `exec` appear to return `(no output)` even for simple commands. This change: - disables detached mode on **darwin** (same as existing win32 behavior) - keeps detached mode enabled on Linux/other POSIX - adds a focused unit test for platform policy (`shouldUseDetached`) ## Validation - `corepack pnpm -s vitest run src/process/supervisor/adapters/child.test.ts` ✅ ## Risk Low. Scope is limited to process spawn options in child adapter. Linux behavior remains unchanged. <!-- greptile_comment --> <h3>Greptile Summary</h3> Disables detached process spawning on macOS to fix stdout/stderr pipe capture issues in LaunchAgent sessions. The PR extracts the platform-specific detached-mode logic into a testable `shouldUseDetached()` function that returns false for both `win32` and `darwin`, while keeping Linux/POSIX behavior unchanged (detached=true with fallback). The fix mirrors the existing Windows Scheduled Task compatibility pattern and includes focused unit test coverage. <h3>Confidence Score: 5/5</h3> - Safe to merge - targeted fix with clear test coverage and no breaking changes to existing platforms - The change is minimal, well-scoped, and directly addresses a documented macOS issue. The new `shouldUseDetached()` helper improves testability, tests cover all three platform cases, and Linux behavior remains unchanged. The fallback mechanism is already in place via `spawnWithFallback()` - No files require special attention _{Last reviewed commit: 4c67157} <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} <!-- /greptile_comment -->