refactor(forge): split forge-interaction into specialized GitHub and Gitea skills

Break the monolithic forge-interaction skill into focused, single-responsibility
skills for better predictability and maintainability:

- Add forge-github skill for GitHub-specific interactions using `gh` CLI
- Add forge-gitea skill for Gitea-specific interactions using `tea` CLI
- Convert forge-interaction into a router that delegates to specialized skills
- Add forge-router skill with model invocation disabled for guidance only
- Move all forge-related skills to common/deprecated directory

Each specialized skill now contains its own complete decision tree, CLI usage
patterns, and authentication checks for its respective forge platform.

common/deprecated/forge-router/SKILL.md

@@ -0,0 +1,50 @@
+---
+name: forge-router
+disable-model-invocation: true
+---
+
+# Forge Router
+
+Choose the right forge interaction skill for your task.
+
+## When to use:
+
+- **GitHub**: Use `/forge-github` when working with GitHub repositories, issues, pull requests, releases, or CI.
+- **Gitea**: Use `/forge-gitea` when working with Gitea repositories, issues, pull requests, releases, or CI.
+
+## How it works:
+
+The router delegates to specialized skills that handle each forge type separately. This keeps each skill focused and predictable.
+
+**GitHub skill** (`/forge-github`):
+- Handles GitHub-specific interactions using the `gh` CLI
+- Follows GitHub's API patterns and conventions
+- Optimized for GitHub's feature set
+
+**Gitea skill** (`/forge-gitea`):
+- Handles Gitea-specific interactions using the `tea` CLI  
+- Follows Gitea's API patterns and conventions
+- Optimized for Gitea's feature set
+
+## Why separate:
+
+- **Predictability**: Each skill knows exactly one forge type
+- **Maintainability**: Changes to GitHub or Gitea logic stay isolated
+- **Clarity**: Users can see which forge a skill handles at a glance
+- **Testing**: Each skill can be tested independently
+
+## Usage examples:
+
+```
+# For GitHub work
+/forge-github create an issue in this repo
+/forge-github list pull requests
+/forge-github check CI status
+
+# For Gitea work  
+/forge-gitea create an issue in this repo
+/forge-gitea list pull requests
+/forge-gitea check CI status
+```
+
+The router ensures you always use the right tool for the right forge.