feat(schema): refine and expand extension metadata schema

- Introduce `scope`, `tags`, `min_version`, `homepage`, and `installation` fields for enhanced discoverability and guidance.
- Remove `install` and `compatibility` objects, replacing them with a simplified `installation` string field.
- Add `examples/` directory with full exemplar files for each extension type.
- Update `schema-design.md` to reflect new schema, clarify field definitions, and improve documentation structure.

examples/agent.yaml

@@ -0,0 +1,89 @@
+# Example Agent Entry
+# Agents are custom AI configurations and skills for specialized tasks.
+# This category includes both agent configs and reusable skills.
+# Place actual entries in: data/agents/your-agent-name.yaml
+
+# ============================================================================
+# REQUIRED FIELDS
+# ============================================================================
+
+name: Example Security Reviewer
+repo: https://github.com/example/opencode-security-agent
+tagline: AI agent specialized in security code review
+description: |
+  A custom agent configuration for security-focused code review.
+  Includes:
+  - Custom system prompts for security analysis
+  - Predefined tools for vulnerability scanning
+  - Skills for OWASP Top 10 detection
+
+# ============================================================================
+# OPTIONAL FIELDS
+# ============================================================================
+
+# Scope defines where the extension can be installed (defaults to [global])
+# Agents can be installed globally or per-project
+scope:
+  - global   # ~/.config/opencode/agent/
+  - project  # .opencode/agent/
+
+tags:
+  - security
+  - code-review
+  - agent
+  - skill
+
+min_version: "1.0.0"
+
+homepage: https://example.com/docs/security-agent
+
+installation: |
+  ## Prerequisites
+  - OpenCode 1.0.0 or later
+
+  ## Installation
+
+  ### Global (all projects)
+  ```bash
+  # Copy agent configuration
+  mkdir -p ~/.config/opencode/agent/security-reviewer
+  curl -o ~/.config/opencode/agent/security-reviewer/agent.json \
+    https://raw.githubusercontent.com/example/opencode-security-agent/main/agent.json
+  ```
+
+  ### Project-only
+  ```bash
+  # Copy to project agent directory
+  mkdir -p .opencode/agent/security-reviewer
+  curl -o .opencode/agent/security-reviewer/agent.json \
+    https://raw.githubusercontent.com/example/opencode-security-agent/main/agent.json
+  ```
+
+  ## Installing Skills
+  If this agent includes skills:
+  ```bash
+  # Global skill
+  mkdir -p ~/.config/opencode/skill/security-review
+  curl -o ~/.config/opencode/skill/security-review/SKILL.md \
+    https://raw.githubusercontent.com/example/opencode-security-agent/main/SKILL.md
+  ```
+
+  ## Configuration
+  Add to your `opencode.json`:
+  ```json
+  {
+    "agents": {
+      "security-reviewer": {
+        "enabled": true
+      }
+    }
+  }
+  ```
+
+  ## Files Modified
+  - `~/.config/opencode/agent/security-reviewer/agent.json`
+  - `~/.config/opencode/skill/security-review/SKILL.md` (if using skills)
+
+  ## Removal
+  1. Delete the agent directory
+  2. Remove config entry from `opencode.json`