feat: add prerequisites field to skill spec — hide skills with unmet dependencies

Skills can now declare runtime prerequisites (env vars, CLI binaries) via
YAML frontmatter. Skills with unmet prerequisites are excluded from the
system prompt so the agent never claims capabilities it can't deliver, and
skill_view() warns the agent about what's missing.

Three layers of defense:
- build_skills_system_prompt() filters out unavailable skills
- _find_all_skills() flags unmet prerequisites in metadata
- skill_view() returns prerequisites_warning with actionable details

Tagged 12 bundled skills that have hard runtime dependencies:
gif-search (TENOR_API_KEY), notion (NOTION_API_KEY), himalaya, imessage,
apple-notes, apple-reminders, openhue, duckduckgo-search, codebase-inspection,
blogwatcher, songsee, mcporter.

Closes #658
Fixes #630

tools/skills_tool.py

@@ -34,6 +34,11 @@ SKILL.md Format (YAML Frontmatter, agentskills.io compatible):
     platforms: [macos]            # Optional — restrict to specific OS platforms
                                   #   Valid: macos, linux, windows
                                   #   Omit to load on all platforms (default)
+    prerequisites:                # Optional — runtime requirements
+      env_vars: [API_KEY]         #   Env vars that must be set (checked via os.getenv)
+      commands: [curl, jq]        #   CLI binaries that must be on PATH (checked via shutil.which)
+                                  #   Skills with unmet prerequisites are hidden from the
+                                  #   system prompt and flagged with a warning in skill_view.
     compatibility: Requires X     # Optional (agentskills.io)
     metadata:                     # Optional, arbitrary key-value (agentskills.io)
       hermes:
@@ -65,6 +70,7 @@ Usage:
 import json
 import os
 import re
+import shutil
 import sys
 from pathlib import Path
 from typing import Dict, Any, List, Optional, Tuple
@@ -118,6 +124,43 @@ def skill_matches_platform(frontmatter: Dict[str, Any]) -> bool:
     return False
 
 
+def check_skill_prerequisites(frontmatter: Dict[str, Any]) -> Tuple[bool, List[str]]:
+    """Check if a skill's declared prerequisites are satisfied.
+
+    Skills declare prerequisites via a top-level ``prerequisites`` dict
+    in their YAML frontmatter::
+
+        prerequisites:
+          env_vars: [TENOR_API_KEY]
+          commands: [curl, jq]
+
+    Returns:
+        (all_met, missing) — True + empty list if all met, else False + list
+        of human-readable descriptions of what's missing.
+    """
+    prereqs = frontmatter.get("prerequisites")
+    if not prereqs or not isinstance(prereqs, dict):
+        return True, []
+
+    missing: List[str] = []
+
+    env_vars = prereqs.get("env_vars") or []
+    if isinstance(env_vars, str):
+        env_vars = [env_vars]
+    for var in env_vars:
+        if not os.getenv(str(var)):
+            missing.append(f"env ${var}")
+
+    commands = prereqs.get("commands") or []
+    if isinstance(commands, str):
+        commands = [commands]
+    for cmd in commands:
+        if not shutil.which(str(cmd)):
+            missing.append(f"command `{cmd}`")
+
+    return (len(missing) == 0), missing
+
+
 def check_skills_requirements() -> bool:
     """Skills are always available -- the directory is created on first use if needed."""
     return True
@@ -262,12 +305,19 @@ def _find_all_skills() -> List[Dict[str, Any]]:
                 description = description[:MAX_DESCRIPTION_LENGTH - 3] + "..."
             
             category = _get_category_from_path(skill_md)
-            
-            skills.append({
+
+            prereqs_met, prereqs_missing = check_skill_prerequisites(frontmatter)
+
+            entry = {
                 "name": name,
                 "description": description,
                 "category": category,
-            })
+            }
+            if not prereqs_met:
+                entry["prerequisites_met"] = False
+                entry["prerequisites_missing"] = prereqs_missing
+
+            skills.append(entry)
             
         except Exception:
             continue
@@ -635,6 +685,17 @@ def skill_view(name: str, file_path: str = None, task_id: str = None) -> str:
             "usage_hint": "To view linked files, call skill_view(name, file_path) where file_path is e.g. 'references/api.md' or 'assets/config.yaml'" if linked_files else None
         }
         
+        # Prerequisite check — warn the agent if requirements are unmet
+        prereqs_met, prereqs_missing = check_skill_prerequisites(frontmatter)
+        if not prereqs_met:
+            result["prerequisites_met"] = False
+            result["prerequisites_missing"] = prereqs_missing
+            result["prerequisites_warning"] = (
+                f"This skill requires {', '.join(prereqs_missing)} which "
+                f"{'is' if len(prereqs_missing) == 1 else 'are'} not available. "
+                f"Tell the user what's needed before attempting to use this skill."
+            )
+
         # Surface agentskills.io optional fields when present
         if frontmatter.get('compatibility'):
             result["compatibility"] = frontmatter['compatibility']