Add feature flags and comprehensive tests

- MORPH_EDIT, MORPH_WARPGREP, MORPH_COMPACT env vars (default true,
  set to "false" to disable). Tools and hooks are conditionally
  registered based on flags.
- 23 new tests for compaction helpers (serializePart,
  messagesToCompactInput, estimateTotalChars, hashMessageIds) and
  integration test that calls live Morph compact API.
- ASCII architecture diagrams in README for all three features.
- 57 tests total, all passing.