chore: wire hxprobe into root project (Makefile, CLAUDE.md, .gitignore)

Makefile: add hx-* targets (deps, run, lint, fmt, test, test-integration,
check, clean) that delegate into hxprobe/ via uv; fold hx-test and hx-check
into the umbrella test/check/clean targets; add HX_DIR variable and a note
that hxprobe is intentionally standalone (no cross-imports from python/).

CLAUDE.md: add "Implementation Summaries" convention — after each feature,
save a summary under docs/summaries/ with the same timestamp+slug naming as
plans. Add note about copying plan-mode scratch files into docs/plans/ before
starting implementation.

.gitignore: add *.egg-info/, .pytest_cache/, .ruff_cache/ for the hxprobe
uv+ruff+pytest toolchain.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -8,6 +8,9 @@ __pycache__/
 *.pyc
 *.pyo
 .venv/
+*.egg-info/
+.pytest_cache/
+.ruff_cache/
 
 # OS
 .DS_Store