McpTool: auto-inject session_id + normalize navi-3d paths ...

- McpTool.execute() now forces the real session_id from current_session_id
  ContextVar, preventing LLM hallucinations of wrong UUIDs (ghost-session bug).
- For navi-3d MCP server, source_path/output_path are normalized to basename
  to prevent double path nesting when the LLM passes full relative paths.
- Updated MCP tool descriptions to ask for filenames only.
- Added system prompt instructions in context_builder and subagent_runner
  reminding the model to pass bare filenames to navi-3d tools.

396 passed, 1 skipped

Step 4: Extract SubAgentRunner from run_ephemeral() ...

- Create navi/core/subagent_runner.py with full sub-agent loop logic
- Move _iter_stream_guarded to navi/core/stream_guard.py
- Move _check_context_size to ContextCompressor.check_context_size()
- Extract build_tool_list() and load_user_enabled_tools() to tool_utils.py
- Agent.run_ephemeral() becomes a thin wrapper delegating to SubAgentRunner
- Remove ~310 lines from agent.py
- All existing run_ephemeral tests pass unchanged

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>