Add unit tests for ContextManager and utils.py (zero coverage)

[wittekin]

Problem

Two modules have no dedicated unit tests:

src/millstone/runtime/context.py — ContextManager

• 3 public methods: get_group_context, accumulate_group_context, extract_context_summary
• ~248 lines; zero test coverage
• Cross-task context sharing is a key feature that could silently break

src/millstone/utils.py

• Core utility functions used throughout the orchestrator:
  • is_empty_response (all schema types + edge cases)
  • extract_claude_result (JSON wrapper extraction, non-JSON passthrough)
  • summarize_output (short/long text)
  • filter_reasoning_traces (Codex thinking block removal)
  • summarize_diff (multi-file diffs, empty input)
  • is_whitespace_or_comment_only_change
  • progress() (BrokenPipeError handling)
• Zero unit tests; only tested indirectly through integration

Fix

Create:

• tests/unit/test_context_manager.py (~100 LoC): mock callbacks, test empty diffs, missing group files, LLM extraction failures
• tests/unit/test_utils.py (~150 LoC): one test per function, covering main paths and edge cases