feat(skills): add fitness-nutrition skill to optional-skills

Cherry-picked from PR #9177 by @haileymarshall.

Adds a fitness and nutrition skill for gym-goers and health-conscious users:
- Exercise search via wger API (690+ exercises, free, no auth)
- Nutrition lookup via USDA FoodData Central (380K+ foods, DEMO_KEY fallback)
- Offline body composition calculators (BMI, TDEE, 1RM, macros, body fat %)
- Pure stdlib Python, no pip dependencies

Changes from original PR:
- Moved from skills/ to optional-skills/health/ (correct location)
- Fixed BMR formula in FORMULAS.md (removed confusing -5+10, now just +5)
- Fixed author attribution to match PR submitter
- Marked USDA_API_KEY as optional (DEMO_KEY works without signup)

Also adds optional env var support to the skill readiness checker:
- New 'optional: true' field in required_environment_variables entries
- Optional vars are preserved in metadata but don't block skill readiness
- Optional vars skip the CLI capture prompt flow
- Skills with only optional missing vars show as 'available' not 'setup_needed'

optional-skills/health/fitness-nutrition/references/FORMULAS.md

@@ -0,0 +1,100 @@
+# Formulas Reference
+
+Scientific references for all calculators used in the fitness-nutrition skill.
+
+## BMI (Body Mass Index)
+
+**Formula:** BMI = weight (kg) / height (m)²
+
+| Category     | BMI Range  |
+|-------------|------------|
+| Underweight | < 18.5     |
+| Normal      | 18.5 – 24.9 |
+| Overweight  | 25.0 – 29.9 |
+| Obese       | 30.0+      |
+
+**Limitation:** BMI does not distinguish muscle from fat. A muscular person
+can have a high BMI while being lean. Use body fat % for a better picture.
+
+Reference: Quetelet, A. (1832). Keys et al., Int J Obes (1972).
+
+## TDEE (Total Daily Energy Expenditure)
+
+Uses the **Mifflin-St Jeor equation** — the most accurate BMR predictor for
+the general population according to the ADA (2005).
+
+**BMR formulas:**
+
+- Male: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
+- Female: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
+
+**Activity multipliers:**
+
+| Level | Description                    | Multiplier |
+|-------|--------------------------------|------------|
+| 1     | Sedentary (desk job)           | 1.200      |
+| 2     | Lightly active (1-3 days/wk)   | 1.375      |
+| 3     | Moderately active (3-5 days)   | 1.550      |
+| 4     | Very active (6-7 days)         | 1.725      |
+| 5     | Extremely active (2x/day)      | 1.900      |
+
+Reference: Mifflin et al., Am J Clin Nutr 51, 241-247 (1990).
+
+## One-Rep Max (1RM)
+
+Three validated formulas. Average of all three is most reliable.
+
+- **Epley:** 1RM = w × (1 + r/30)
+- **Brzycki:** 1RM = w × 36 / (37 − r)
+- **Lombardi:** 1RM = w × r^0.1
+
+All formulas are most accurate for r ≤ 10. Above 10 reps, error increases.
+
+Reference: LeSuer et al., J Strength Cond Res 11(4), 211-213 (1997).
+
+## Macro Splits
+
+Recommended splits based on goal:
+
+| Goal         | Protein | Fat  | Carbs | Calorie Offset |
+|-------------|---------|------|-------|----------------|
+| Fat loss    | 40%     | 30%  | 30%   | −500 kcal      |
+| Maintenance | 30%     | 30%  | 40%   | 0              |
+| Lean bulk   | 30%     | 25%  | 45%   | +400 kcal      |
+
+Protein targets for muscle growth: 1.6–2.2 g/kg body weight per day.
+Minimum fat intake: 0.5 g/kg to support hormone production.
+
+Conversion: Protein = 4 kcal/g, Fat = 9 kcal/g, Carbs = 4 kcal/g.
+
+Reference: Morton et al., Br J Sports Med 52, 376–384 (2018).
+
+## Body Fat % (US Navy Method)
+
+**Male:**
+
+BF% = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
+
+**Female:**
+
+BF% = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
+
+All measurements in centimeters.
+
+| Category      | Male   | Female |
+|--------------|--------|--------|
+| Essential    | 2-5%   | 10-13% |
+| Athletic     | 6-13%  | 14-20% |
+| Fitness      | 14-17% | 21-24% |
+| Average      | 18-24% | 25-31% |
+| Obese        | 25%+   | 32%+   |
+
+Accuracy: ±3-5% compared to DEXA. Measure at the navel (waist),
+at the Adam's apple (neck), and widest point (hip, females only).
+
+Reference: Hodgdon & Beckett, Naval Health Research Center (1984).
+
+## APIs
+
+- wger: https://wger.de/api/v2/ — AGPL-3.0, exercise data is CC-BY-SA 3.0
+- USDA FoodData Central: https://api.nal.usda.gov/fdc/v1/ — public domain (CC0 1.0)