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/scripts/nutrition_search.py

@@ -0,0 +1,86 @@
+#!/usr/bin/env python3
+"""
+nutrition_search.py — Search USDA FoodData Central for nutrition info.
+
+Usage:
+  python3 nutrition_search.py "chicken breast"
+  python3 nutrition_search.py "rice" "eggs" "broccoli"
+  echo -e "oats\\nbanana\\nwhey protein" | python3 nutrition_search.py -
+
+Reads USDA_API_KEY from environment, falls back to DEMO_KEY.
+No external dependencies.
+"""
+import sys
+import os
+import json
+import time
+import urllib.request
+import urllib.parse
+import urllib.error
+
+API_KEY = os.environ.get("USDA_API_KEY", "DEMO_KEY")
+BASE = "https://api.nal.usda.gov/fdc/v1"
+
+
+def search(query, max_results=3):
+    encoded = urllib.parse.quote(query)
+    url = (
+        f"{BASE}/foods/search?api_key={API_KEY}"
+        f"&query={encoded}&pageSize={max_results}"
+        f"&dataType=Foundation,SR%20Legacy"
+    )
+    try:
+        req = urllib.request.Request(url, headers={"Accept": "application/json"})
+        with urllib.request.urlopen(req, timeout=15) as r:
+            return json.loads(r.read())
+    except Exception as e:
+        print(f"  API error: {e}", file=sys.stderr)
+        return None
+
+
+def display(food):
+    nutrients = {n["nutrientName"]: n.get("value", "?") for n in food.get("foodNutrients", [])}
+    cal = nutrients.get("Energy", "?")
+    prot = nutrients.get("Protein", "?")
+    fat = nutrients.get("Total lipid (fat)", "?")
+    carb = nutrients.get("Carbohydrate, by difference", "?")
+    fib = nutrients.get("Fiber, total dietary", "?")
+    sug = nutrients.get("Sugars, total including NLEA", "?")
+
+    print(f"  {food.get('description', 'N/A')}")
+    print(f"    Calories : {cal} kcal")
+    print(f"    Protein  : {prot}g")
+    print(f"    Fat      : {fat}g")
+    print(f"    Carbs    : {carb}g (fiber: {fib}g, sugar: {sug}g)")
+    print(f"    FDC ID   : {food.get('fdcId', 'N/A')}")
+
+
+def main():
+    if len(sys.argv) < 2:
+        print(__doc__)
+        sys.exit(1)
+
+    if sys.argv[1] == "-":
+        queries = [line.strip() for line in sys.stdin if line.strip()]
+    else:
+        queries = sys.argv[1:]
+
+    for query in queries:
+        print(f"\n--- {query.upper()} (per 100g) ---")
+        data = search(query, max_results=2)
+        if not data or not data.get("foods"):
+            print("  No results found.")
+        else:
+            for food in data["foods"]:
+                display(food)
+                print()
+        if len(queries) > 1:
+            time.sleep(1)  # respect rate limits
+
+    if API_KEY == "DEMO_KEY":
+        print("\nTip: using DEMO_KEY (30 req/hr). Set USDA_API_KEY for 1000 req/hr.")
+        print("Free signup: https://fdc.nal.usda.gov/api-key-signup/")
+
+
+if __name__ == "__main__":
+    main()