Add Obsidian installer - Knowledge management tool

[gdellis]

Summary

This PR adds a comprehensive installer for Obsidian, a powerful knowledge base that works on local Markdown files.

Features

• Multiple installation formats: AppImage (default), Deb, Snap
• Smart version detection: Auto-detect latest version from GitHub releases via API
• Version specification: Install specific version with --version flag
• Format flexibility: Choose installation format with --format flag
• Configuration backup: Backs up existing configuration before modifications
• Desktop integration: Creates proper .desktop entry for application menus
• Comprehensive help: Full help documentation with examples

Installation Options

# Install latest version as AppImage (default)
./installers/setup_obsidian.sh

# Install specific version as Deb package
./installers/setup_obsidian.sh --version 1.11.7 --format deb

# Install as Snap package
./installers/setup_obsidian.sh --format snap --non-interactive

# Show help
./installers/setup_obsidian.sh --help

Pre-PR Requirements (All Met)

• Installer structure validated ✓ (18/18 installers now pass)
• ShellCheck passes locally ✓ (severity=error)
• Commit follows conventional format ✓
• Branch is up-to-date with main ✓

Testing Performed

# Validation
./validate_installers.sh
Total installers: 18
Passed: 18 ✓
Failed: 0

# ShellCheck
shellcheck --severity=error --external-sources installers/setup_obsidian.sh
# (no output = all good)

Technical Details

• Download source: Official GitHub releases (https://github.com/obsidianmd/obsidian-releases)
• API integration: Uses GitHub API to auto-detect latest version
• Fallback mechanism: Hardcoded fallback version (v1.11.7) if API fails
• Checksum verification: Ready for future checksum validation
• Cross-platform: Supports all Linux distributions with multiple formats

Installation Flow

1. Detect latest version (or use specified version)
2. Download selected format (AppImage/Deb/Snap)
3. Install based on format type
4. Create desktop entry for system integration
5. Backup existing configuration if present
6. Provide helpful post-install information

Files Changed

• installers/setup_obsidian.sh (265 lines)

This installer follows all project standards and provides a robust installation experience for Obsidian users.