Add installer check and dry-run modes

[barbatdev]

Context\n\nThe Gentleman.Dots inventory showed strong value in installer modes that can validate and preview changes before touching the machine. Cortex-dotfiles currently manages shell, terminal, Herdr, Claude/OpenCode, macOS services, fonts and symlinks, so a read-only check path would reduce risk.\n\n## Goal\n\nAdd safe installer modes:\n\n- ./install.sh --check: read-only validation of tools, symlinks, config files, integrations and services.\n- ./install.sh --dry-run: print planned backups, symlinks and installs without mutating.\n\n## Scope\n\n- Keep normal ./install.sh behavior unchanged.\n- Reuse existing validation logic where possible.\n- Report missing tools/configs clearly.\n- No destructive actions in either mode.\n\n## Validation\n\n- bash -n install.sh\n- ./install.sh --check exits 0 or with actionable diagnostics.\n- ./install.sh --dry-run does not change existing symlinks/files.