When multiple `sessionStart`/`subagentStart` hooks output `additionalContext`, only the last one is injected into the context

### Describe the bug

I have multiple sessionStart/subagentStart hooks, and each of them prints `{"additionalContext":"some value"}` to stdout. I observed, that only the last emitted value is actually injected into the context. I validated this by simulating this with 2 synthetic hooks that inject different things and then asking the agent about them (or telling it to spawn a sub-agent and ask it). In all cases, only one of the two values is known to the agent.

To be clear, the hooks are all called (I validated this by adding an `echo >> /tmp/test.txt` to each hook), so the problem is not that only one of them is run. All are run, but only the last output is injected.

### Affected version

GitHub Copilot CLI 1.0.56.

### Steps to reproduce the behavior

1. Define multiple `sessionStart`/`subagentStart` hooks that each print `additionalContext` to stdout
2. Start a new copilot instance
3. Ask it about both the injected contexts.

### Expected behavior

If multiple hooks return additional context, they should all be concatenated.

### Additional context

_No response_

Start by reproducing the issue with two sessionStart or subagentStart hooks that each emit different additionalContext values. Trace how hook outputs are collected and injected, then verify that the completed behavior makes both values available to the agent rather than only the last one.

Written by the indexing model from the issue text.