Hook additionalContext: Structured JSON Output

Hooks that return exit code 0 now communicate with Claude through a structured additionalContext JSON field instead of plain stdout. This change applies to SubagentStart, PreToolUse, PostToolUse, and session-init hooks.

Before: Exit code 0 → stdout string shown to Claude/subagent directly.

After: Exit code 0 → stdout parsed as JSON; the additionalContext field from the parsed object is injected into Claude's context. Exit code 2 now routes stderr to the user only (previously stderr was ignored for blocking errors).

Hook descriptions updated:

Exit code 0 - JSON additionalContext shown to subagent
Exit code 2 - show stderr to user only
Other exit codes - show stderr to user only