feat(installer): align install paths with platform standards and improve shell guidance

[CalvinAllen]

Add XDG_DATA_HOME support on Linux and document platform path rationale.

Current State

• ✅ Shell-specific PATH guidance implemented for bash, zsh, fish, PowerShell
• ✅ Platform-specific path output after installation
• ❌ No XDG_DATA_HOME support on Linux (hardcoded ~/.dtvem)
• ❌ No documentation of path choice rationale

Remaining Work

Linux XDG Support

• Check XDG_DATA_HOME environment variable on Linux
• Default to $XDG_DATA_HOME/dtvem if set, otherwise ~/.dtvem
• Update install.sh to respect XDG paths
• Update src/internal/config/paths.go for runtime XDG support

Documentation

• Document rationale for Windows path choice (%USERPROFILE%\.dtvem vs %LOCALAPPDATA%)
• Document BSD/macOS path conventions
• Add troubleshooting section for PATH issues

Testing

• Unit tests for path selection logic with XDG_DATA_HOME set/unset
• Integration test for at least one shell per platform

Acceptance Criteria

• Linux respects XDG_DATA_HOME when set
• Path rationale documented for all platforms
• Output guidance accurate for bash, zsh, fish, PowerShell, cmd.exe