Add ID space partitioning and improve auto-flush reliability

Three improvements to beads:

1. ID space partitioning (closes bd-24)
   - Add --id flag to 'bd create' for explicit ID assignment
   - Validates format: prefix-number (e.g., worker1-100)
   - Enables parallel agents to partition ID space and avoid conflicts
   - Storage layer already supported this, just wired up CLI

2. Auto-flush failure tracking (closes bd-38)
   - Track consecutive flush failures with counter and last error
   - Show prominent red warning after 3+ consecutive failures
   - Reset counter on successful flush
   - Users get clear guidance to run manual export if needed

3. Manual export cancels auto-flush timer
   - Add clearAutoFlushState() helper function
   - bd export now cancels pending auto-flush and clears dirty flag
   - Prevents redundant exports when user manually exports
   - Also resets failure counter on successful manual export

Documentation updated in README.md and CLAUDE.md with --id flag examples.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

CLAUDE.md

@@ -17,6 +17,9 @@ bd ready --json
 # Create new issue
 bd create "Issue title" -t bug|feature|task -p 0-4 -d "Description" --json
 
+# Create with explicit ID (for parallel workers)
+bd create "Issue title" --id worker1-100 -p 1 --json
+
 # Update issue status
 bd update <id> --status in_progress --json
 
@@ -236,6 +239,7 @@ bd dep tree bd-8  # Show 1.0 epic dependencies
 - Priority 0-1 issues are usually more important than 2-4
 - Use `--dry-run` to preview import collisions before resolving
 - Use `--resolve-collisions` for safe automatic branch merges
+- Use `--id` flag with `bd create` to partition ID space for parallel workers (e.g., `worker1-100`, `worker2-500`)
 
 ## Building and Testing