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>

CLAUDE.md

@@ -10,6 +10,23 @@ a short kebab-case description.
 
 Example: `docs/plans/2026-07-01-00-08-go-latency-tool.md`
 
+If a plan is drafted via plan-mode tooling that restricts writes to a separate
+scratch file, copy the approved plan verbatim into `docs/plans/` as the first
+implementation step, before making any code changes. The scratch file existing
+elsewhere does not satisfy this convention.
+
+## Implementation Summaries
+
+After finishing the implementation of a feature, save a summary under
+`docs/summaries/` with a filename that starts with the **current timestamp in
+`yyyy-mm-dd-hh-mm` format** followed by a short kebab-case description (same
+convention as plans). The summary covers what was actually built (as opposed
+to the plan, which covers what was intended): files added/changed, key design
+decisions, any deviations from the plan, notable findings made along the way,
+and how it was verified.
+
+Example: `docs/summaries/2026-07-02-00-29-py-hxprobe-httpx.md`
+
 ## Changelog
 
 Every completed feature is appended to `CHANGELOG.md` at the project root with