Runtimes like Antigravity don't have a Task tool for spawning subagents.
When the agent encounters Task() calls, it falls back to browser_subagent
which is meant for web browsing, not code analysis — causing
gsd-map-codebase to fail.
This adds:
1. A detect_runtime_capabilities step before spawn_agents
2. An explicit warning to NEVER use browser_subagent for code analysis
3. A sequential_mapping fallback step that performs all 4 mapping passes
inline using file system tools when Task is unavailable
Closes#1174
9acfa4bffc