feat: DAG-based tool execution with concurrent_safe split + id uniquification

[Simon-Free]

Changes

• dag.py (new): DAG-based parallel tool execution with topological ordering and depends_on support
• id_uniquify.py (new): Prevents GC auto-stubbing collisions when model reuses short IDs across turns
• agent.py: Integrates DAG executor and id uniquification into the main loop
• tool_registry.py: Adds is_concurrent_safe() helper

Tests

• 12 tests for DAG concurrency (parallel, dependency chains, mixed safe/unsafe)
• 10 tests for ID uniquification (collision detection, remapping, idempotency)

Port of bouzecode agent loop improvements.