feat(v2.3.0): Add multi-agent installation support

Add automatic skill installation to 10+ AI coding agents with a single command.

New Features:
- New install-agent command for installing skills to any AI agent
- Support for 10+ agents: Claude Code, Cursor, VS Code, Amp, Goose, OpenCode, Letta, Aide, Windsurf
- Smart path resolution (global ~/.agent vs project-relative .agent/)
- Fuzzy agent name matching with suggestions
- --agent all flag to install to all agents at once
- --force flag to overwrite existing installations
- --dry-run flag to preview installations
- Comprehensive error handling and user feedback

Implementation:
- Created install_agent.py (379 lines) with core installation logic
- Updated main.py with install-agent subcommand
- Updated pyproject.toml with entry point
- Added 32 comprehensive tests (all passing, 603 total)
- No regressions in existing functionality

Documentation:
- Updated README.md with multi-agent installation guide
- Updated CLAUDE.md with install-agent examples
- Updated CHANGELOG.md with v2.3.0 release notes
- Added agent compatibility table

Technical Details:
- 100% own implementation (no external dependencies)
- Pure Python using stdlib (shutil, pathlib, argparse)
- Compatible with Agent Skills open standard (agentskills.io)
- Works offline

Closes #210

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -321,6 +321,28 @@ skill-seekers upload output/godot.zip
 skill-seekers package output/godot/ --no-open
 ```
 
+### Install to AI Agents
+
+```bash
+# Single agent installation
+skill-seekers install-agent output/godot/ --agent cursor
+
+# Install to all agents
+skill-seekers install-agent output/godot/ --agent all
+
+# Force overwrite
+skill-seekers install-agent output/godot/ --agent claude --force
+
+# Dry run (preview only)
+skill-seekers install-agent output/godot/ --agent cursor --dry-run
+```
+
+**Supported agents:** claude, cursor, vscode, copilot, amp, goose, opencode, letta, aide, windsurf, all
+
+**Installation paths:**
+- Global agents (claude, amp, goose, etc.): Install to `~/.{agent}/skills/`
+- Project agents (cursor, vscode): Install to `.{agent}/skills/` in current directory
+
 ### Force Re-scrape
 
 ```bash