docs: fix stale env-var docs, document validateProfile's zone-range rationale

Two Minor findings from the final whole-branch review: the smartrun-dev
skill still told developers to set GARMIN_EMAIL/GARMIN_PASSWORD (now
sourced from the profile row instead), and validateProfile's deliberate
choice not to require 0-100% HR zone coverage had no explanation.

.claude/skills/smartrun-dev/SKILL.md

@@ -75,7 +75,7 @@ Each `workout_kinds.rule_json` is a recursive AND/OR condition tree (`internal/c
 
 ## Dev workflow
 
-- Backend: `cd backend && go run ./cmd/smartrund` (needs `MCP_GARMIN_PYTHON`, `MCP_GARMIN_SERVER`, `GARMIN_EMAIL`, `GARMIN_PASSWORD` env vars; see `internal/config/config.go` for all knobs).
+- Backend: `cd backend && go run ./cmd/smartrund` (needs `MCP_GARMIN_PYTHON`/`MCP_GARMIN_SERVER` env vars for the mcp-garmin subprocess paths; see `internal/config/config.go` for all knobs). Garmin credentials are **not** env vars — they live in the `profile` singleton row (`internal/store.Profile`), set via the frontend's Profile page or directly through `PUT /api/profile`.
 - Frontend: `cd frontend && npm run dev` (set `VITE_API_BASE_URL` if the backend isn't on `localhost:8080`).
 - No live Garmin account needed for frontend/UI work: `go run ./cmd/seedsample -db /tmp/sample.db` seeds realistic activities/laps/kinds and runs them through the real classification engine, then point `smartrund` at that DB.
 - `internal/garmin/mock` provides a fake `Client` for tests that need to exercise `internal/sync`/`internal/api` without a live subprocess.

backend/internal/api/profile.go

@@ -8,6 +8,11 @@ import (
 	"smartrun/backend/internal/store"
 )
 
+// validateProfile checks the HR zones are ascending and gap-free once they
+// start, but deliberately does NOT require zone 1 to start at 0% or zone 5
+// to end at 100%: real Karvonen HR training zones (e.g. the seeded defaults,
+// 50-60/60-70/70-80/80-90/90-100) don't cover the 0-50% range at all --
+// below zone 1 simply isn't a named training zone.
 func validateProfile(p store.Profile) error {
 	if p.RestingHeartRate != nil && p.MaxHeartRate != nil && *p.RestingHeartRate >= *p.MaxHeartRate {
 		return errors.New("resting heart rate must be less than max heart rate")