fix: handle multiple lifecycle hooks without closed pipe reuse

[lox]

Problem

The agent startup lifecycle hook runner supports multiple hook paths (HooksPath + AdditionalHooksPaths), but the shared output pipe writer was closed inside the per-hook loop.

That means the first hook could run successfully, then the second hook attempted to write to a closed pipe and failed with:

io: read/write on closed pipe

Impact

When users configure additional lifecycle hook paths, startup/shutdown hooks can fail even though hook scripts themselves are valid.

This is user-visible and can break agent bootstrap behaviour in environments that rely on multiple global hook directories.

Reproduction

Re-enabled TestAgentStartupHookWithAdditionalPaths, which previously failed with the closed-pipe error on the second hook execution.

Fix

• return early when no lifecycle hooks are found (avoid unnecessary scanner goroutine)
• keep the pipe/scanner alive for the whole hook batch
• close the writer and wait for scanner completion once via deferred cleanup, after all hooks have run
• remove per-hook w.Close() / wg.Wait() inside the loop

This preserves streaming log output while preventing closed-pipe reuse across hooks.

Testing

• go test ./clicommand -run 'TestAgentStartupHookWithAdditionalPaths|TestAgentStartupHook|TestAgentShutdownHook' -count=1

[DrJosh9000]

LGTM