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>

README.md

@@ -154,6 +154,9 @@ bd create "Fix bug" -d "Description" -p 1 -t bug
 bd create "Add feature" --description "Long description" --priority 2 --type feature
 bd create "Task" -l "backend,urgent" --assignee alice
 
+# Explicit ID (useful for parallel workers to avoid conflicts)
+bd create "Worker task" --id worker1-100 -p 1
+
 # Get JSON output for programmatic use
 bd create "Fix bug" -d "Description" --json
 ```
@@ -164,6 +167,7 @@ Options:
 - `-t, --type` - Type (bug|feature|task|epic|chore)
 - `-a, --assignee` - Assign to user
 - `-l, --labels` - Comma-separated labels
+- `--id` - Explicit issue ID (e.g., `worker1-100` for ID space partitioning)
 - `--json` - Output in JSON format
 
 ### Viewing Issues