Convert GarminSleep, GarminHRAPI, GarminSPo2 to LAVA @artifact_processor

[abrignoni]

More Garmin family conversions. These had per-row interactive chart buttons (createPieChart/createLineChart) that only function inside the HTML report — dropped them for LAVA, preserving the underlying data.

• GarminSleep — sleepStartTimeGMT/sleepEndTimeGMT (epoch-ms) → aware UTC datetime (replacing SQL datetime(...,'unixepoch')). Dropped the 'Graph' pie-chart button column; its inputs (deep/light/REM/awake seconds) are already their own columns. Removed an unused calendarDate select.
• GarminHRAPI — JSON heart-rate daily summary. Dropped the 'Graphic' line-chart button (and the now-dead HR time-series parsing); kept date + max/min/resting/average HR. Added encoding to open().
• GarminSPo2 — replaced the (buggy, globally-accumulating) 'View' chart button in the 'SPO2 Values Array' column with the actual raw spo2ValuesArray value, so real data is preserved instead of a non-functional button. Kept the GMT timestamps as the app's stored strings.

Verified: byte-compile, decorated 4-arg signatures, return 3-tuples, output_types valid, loader builds (407 plugins), pylint clean.