[BUG] PostToolUse hook stdout/stderr never surfaces in agent context

Describe the bug

PostToolUse hook stdout and stderr are never injected into the agent's context window. The hook fires (confirmed via file side-effects), the script runs, but echo output is silently dropped. This makes PostToolUse hooks useless for injecting reminders, guardrails, or context at the moment a tool completes.

To reproduce

1. Add a PostToolUse hook in .claude/settings.json:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "echo 'CANARY_STRING'"
          }
        ]
      }
    ]
  }
}

1. Run any Bash command (e.g. echo "ping")
2. The hook fires (verified by replacing echo with touch /tmp/test-file — file appears)
3. But CANARY_STRING never appears in the tool result or anywhere the agent can see it

Observed behavior

Tested all combinations:

| Hook type | Exit code | stdout visible? | stderr visible? |
|-------------|-----------|-----------------|-----------------|
| PreToolUse | 0 | No | No |
| PreToolUse | non-zero | No | Yes (as block error) |
| PostToolUse | 0 | No | No |
| PostToolUse | non-zero | No | No |

Only blocking PreToolUse hooks (exit non-zero) surface their stderr to the agent via the error message format: PreToolUse:Bash hook error: [command]: <stderr>.

Expected behavior

PostToolUse hook stdout should be appended to the tool result (or injected as a system message), so the agent can see and act on it. This is the primary use case for PostToolUse hooks — injecting context-sensitive guidance at the moment a specific tool completes.

Workaround

Use a PreToolUse blocking hook (exit 2) that writes to stderr. The agent sees the error, processes the message, and retries the command. Requires a breadcrumb file so it only blocks once per session. Ugly but functional.

Environment

• Claude Code CLI + Conductor (Mac app)
• macOS
• Tested mid-session and with fresh sessions
• Hooks hot-reload mid-session (not a stale-config issue)