update changelog and cheatsheet for sub-sport and dev mode

CHANGELOG.md

@@ -2,6 +2,19 @@
 
 ## [Unreleased] — 2026-03-30
 
+### Sport classification
+
+- **Sub-sport detection** — `normalise_sub_sport()` in `sport.py` infers sub_sport from raw sport type strings
+  - CamelCase Strava types handled correctly (`MountainBikeRide` → `cycling / mountain`, `GravelRide` → `cycling / gravel`, `AlpineSki` → `skiing / alpine`, `NordicSki` → `skiing / nordic`, etc.)
+  - All parsers (Strava importer, GPX, TCX) now populate `sub_sport`; FIT parser was already correct
+  - Sub-sport shown as a secondary pill on activity detail page: **🚴 Cycling** + **MTB**
+
+### Developer experience
+
+- **`--dev N` flag** on `bincio extract` — samples N files evenly across the full file list (date + format diversity) and writes to `/tmp/bincio_dev/`; `incremental` is disabled automatically
+- **`--dev N` flag** on `bincio import strava` — imports only the N most recent activities to `/tmp/bincio_dev/`
+- Dev loop: `bincio extract --dev 50 && bincio import strava --dev 50 && bincio render --serve --data-dir /tmp/bincio_dev`
+
 ### Data ingestion
 
 - **`bincio import strava`** — OAuth2 Strava importer (`bincio/import_/strava.py` + `bincio/import_/cli.py`)