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

skills/apple/apple-notes/SKILL.md

@@ -9,6 +9,8 @@ metadata:
   hermes:
     tags: [Notes, Apple, macOS, note-taking]
     related_skills: [obsidian]
+prerequisites:
+  commands: [memo]
 ---
 
 # Apple Notes

skills/apple/apple-reminders/SKILL.md

@@ -8,6 +8,8 @@ platforms: [macos]
 metadata:
   hermes:
     tags: [Reminders, tasks, todo, macOS, Apple]
+prerequisites:
+  commands: [remindctl]
 ---
 
 # Apple Reminders

skills/apple/imessage/SKILL.md

@@ -8,6 +8,8 @@ platforms: [macos]
 metadata:
   hermes:
     tags: [iMessage, SMS, messaging, macOS, Apple]
+prerequisites:
+  commands: [imsg]
 ---
 
 # iMessage

skills/email/himalaya/SKILL.md

@@ -8,6 +8,8 @@ metadata:
   hermes:
     tags: [Email, IMAP, SMTP, CLI, Communication]
     homepage: https://github.com/pimalaya/himalaya
+prerequisites:
+  commands: [himalaya]
 ---
 
 # Himalaya Email CLI

skills/feeds/blogwatcher/SKILL.md

@@ -8,6 +8,8 @@ metadata:
   hermes:
     tags: [RSS, Blogs, Feed-Reader, Monitoring]
     homepage: https://github.com/Hyaxia/blogwatcher
+prerequisites:
+  commands: [blogwatcher]
 ---
 
 # Blogwatcher

skills/gifs/gif-search/SKILL.md

@@ -1,9 +1,12 @@
 ---
 name: gif-search
 description: Search and download GIFs from Tenor using curl. No dependencies beyond curl and jq. Useful for finding reaction GIFs, creating visual content, and sending GIFs in chat.
-version: 1.0.0
+version: 1.1.0
 author: Hermes Agent
 license: MIT
+prerequisites:
+  env_vars: [TENOR_API_KEY]
+  commands: [curl, jq]
 metadata:
   hermes:
     tags: [GIF, Media, Search, Tenor, API]
@@ -13,32 +16,43 @@ metadata:
 
 Search and download GIFs directly via the Tenor API using curl. No extra tools needed.
 
+## Setup
+
+Set your Tenor API key in your environment (add to `~/.hermes/.env`):
+
+```bash
+TENOR_API_KEY=your_key_here
+```
+
+Get a free API key at https://developers.google.com/tenor/guides/quickstart — the Google Cloud Console Tenor API key is free and has generous rate limits.
+
 ## Prerequisites
 
-- `curl` and `jq` (both standard on Linux)
+- `curl` and `jq` (both standard on macOS/Linux)
+- `TENOR_API_KEY` environment variable
 
 ## Search for GIFs
 
 ```bash
 # Search and get GIF URLs
-curl -s "https://tenor.googleapis.com/v2/search?q=thumbs+up&limit=5&key=AIzaSyAyimkuYQYF_FXVALexPuGQctUWRURdCYQ" | jq -r '.results[].media_formats.gif.url'
+curl -s "https://tenor.googleapis.com/v2/search?q=thumbs+up&limit=5&key=${TENOR_API_KEY}" | jq -r '.results[].media_formats.gif.url'
 
 # Get smaller/preview versions
-curl -s "https://tenor.googleapis.com/v2/search?q=nice+work&limit=3&key=AIzaSyAyimkuYQYF_FXVALexPuGQctUWRURdCYQ" | jq -r '.results[].media_formats.tinygif.url'
+curl -s "https://tenor.googleapis.com/v2/search?q=nice+work&limit=3&key=${TENOR_API_KEY}" | jq -r '.results[].media_formats.tinygif.url'
 ```
 
 ## Download a GIF
 
 ```bash
 # Search and download the top result
-URL=$(curl -s "https://tenor.googleapis.com/v2/search?q=celebration&limit=1&key=AIzaSyAyimkuYQYF_FXVALexPuGQctUWRURdCYQ" | jq -r '.results[0].media_formats.gif.url')
+URL=$(curl -s "https://tenor.googleapis.com/v2/search?q=celebration&limit=1&key=${TENOR_API_KEY}" | jq -r '.results[0].media_formats.gif.url')
 curl -sL "$URL" -o celebration.gif
 ```
 
 ## Get Full Metadata
 
 ```bash
-curl -s "https://tenor.googleapis.com/v2/search?q=cat&limit=3&key=AIzaSyAyimkuYQYF_FXVALexPuGQctUWRURdCYQ" | jq '.results[] | {title: .title, url: .media_formats.gif.url, preview: .media_formats.tinygif.url, dimensions: .media_formats.gif.dims}'
+curl -s "https://tenor.googleapis.com/v2/search?q=cat&limit=3&key=${TENOR_API_KEY}" | jq '.results[] | {title: .title, url: .media_formats.gif.url, preview: .media_formats.tinygif.url, dimensions: .media_formats.gif.dims}'
 ```
 
 ## API Parameters
@@ -47,7 +61,7 @@ curl -s "https://tenor.googleapis.com/v2/search?q=cat&limit=3&key=AIzaSyAyimkuYQ
 |-----------|-------------|
 | `q` | Search query (URL-encode spaces as `+`) |
 | `limit` | Max results (1-50, default 20) |
-| `key` | API key (the one above is Tenor's public demo key) |
+| `key` | API key (from `$TENOR_API_KEY` env var) |
 | `media_filter` | Filter formats: `gif`, `tinygif`, `mp4`, `tinymp4`, `webm` |
 | `contentfilter` | Safety: `off`, `low`, `medium`, `high` |
 | `locale` | Language: `en_US`, `es`, `fr`, etc. |
@@ -67,7 +81,6 @@ Each result has multiple formats under `.media_formats`:
 
 ## Notes
 
-- The API key above is Tenor's public demo key — it works but has rate limits
 - URL-encode the query: spaces as `+`, special chars as `%XX`
 - For sending in chat, `tinygif` URLs are lighter weight
 - GIF URLs can be used directly in markdown: `![alt](url)`

skills/github/codebase-inspection/SKILL.md

@@ -8,6 +8,8 @@ metadata:
   hermes:
     tags: [LOC, Code Analysis, pygount, Codebase, Metrics, Repository]
     related_skills: [github-repo-management]
+prerequisites:
+  commands: [pygount]
 ---
 
 # Codebase Inspection with pygount

skills/mcp/mcporter/SKILL.md

@@ -8,6 +8,8 @@ metadata:
   hermes:
     tags: [MCP, Tools, API, Integrations, Interop]
     homepage: https://mcporter.dev
+prerequisites:
+  commands: [npx]
 ---
 
 # mcporter

skills/music-creation/songsee/SKILL.md

@@ -8,6 +8,8 @@ metadata:
   hermes:
     tags: [Audio, Visualization, Spectrogram, Music, Analysis]
     homepage: https://github.com/steipete/songsee
+prerequisites:
+  commands: [songsee]
 ---
 
 # songsee

skills/productivity/notion/SKILL.md

@@ -8,6 +8,8 @@ metadata:
   hermes:
     tags: [Notion, Productivity, Notes, Database, API]
     homepage: https://developers.notion.com
+prerequisites:
+  env_vars: [NOTION_API_KEY]
 ---
 
 # Notion API

skills/research/duckduckgo-search/SKILL.md

@@ -8,6 +8,8 @@ metadata:
   hermes:
     tags: [search, duckduckgo, web-search, free, fallback]
     related_skills: [arxiv]
+prerequisites:
+  commands: [ddgs]
 ---
 
 # DuckDuckGo Search (Firecrawl Fallback)

skills/smart-home/openhue/SKILL.md

@@ -8,6 +8,8 @@ metadata:
   hermes:
     tags: [Smart-Home, Hue, Lights, IoT, Automation]
     homepage: https://www.openhue.io/cli
+prerequisites:
+  commands: [openhue]
 ---
 
 # OpenHue CLI