fix: auto-create .claude/current-mode if missing for coordination hooks

The UserPromptSubmit hook requires .claude/current-mode to determine work mode
and gate coordination lock checks. This file is machine-local (gitignored) but
had no initialization logic for fresh clones, causing hooks to fail.

Changes:
- check-messages.sh: Added auto-creation logic with "general" as default
- CLAUDE.md: Documented auto-initialization behavior
- ONBOARDING.md: Added machine-local configuration section
- session-logs/2026-05-19-session.md: Documented investigation and fix

Impact:
- Fixes coordination hooks on all machines
- Prevents first-clone hook failures
- No manual setup required
- Backwards compatible

Resolves: "cood hook seems to be broken on all my machines"

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.claude/CLAUDE.md

@@ -67,6 +67,8 @@ echo dev > .claude/current-mode   # substitute the actual mode name
 ```
 This file is gitignored (machine-local). The `UserPromptSubmit` hook reads it to gate the lock check on dev mode.
 
+**Auto-initialization:** If `.claude/current-mode` is missing (e.g., fresh clone), the UserPromptSubmit hook automatically creates it with "general" as the default mode. No manual setup required.
+
 ---
 
 ## Identity: You Are a Coordinator