Plugin SessionStart hooks execute but additionalContext stdout is silently discarded

Problem

Plugin SessionStart hooks execute successfully (side effects work) but their stdout JSON containing hookSpecificOutput.additionalContext is silently discarded. The injected context never appears in the agent conversation.

Stop hooks in the same plugin work correctly (blocking error prompts appear). Only SessionStart additionalContext is affected.

Reproduction

1. Install a plugin with this hooks.json:

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "bash ${CLAUDE_PLUGIN_ROOT}/hooks/session-start.sh",
            "timeout": 10
          }
        ]
      }
    ]
  }
}

1. session-start.sh outputs valid JSON:

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "This text should appear in conversation context"
  }
}

1. Start a new interactive terminal session

Expected: additionalContext text appears as a system-reminder in conversation context
Actual: Hook executes (file side effects work), stdout is discarded, no context injected

Evidence

• Hook execution confirmed via file markers (touch commands in the script create files)
• Plugin loading confirmed via channel MCP tools appearing in deferred tools list
• Manual execution of the script produces valid JSON
• Stop hooks in the same plugin/hooks.json work correctly
• Tested across 4 agents in different project directories, all consistent
• Same hooks.json format that works for settings.json hooks

Environment

• Claude Code latest (2026-04-08)
• Plugin installed at user scope
• macOS Darwin 25.3.0
• Interactive terminal sessions (not --print mode)