implement Phase 1 + 2: idempotent link, apply command

[Marcusk19]

• Rewrite cmd/link.go with safe idempotent logic: skips correct symlinks, backs up real files to .dotctl.bak, respects --overwrite and --no-backup
• Add cmd/apply.go: new bootstrap command that clones or pulls a dotfiles repo then runs the idempotent link logic with a summary
• Add --overwrite and --no-backup persistent flags to root command
• Remove unstable cmd/sync.go
• Fix cmd/init.go gitignore to not exclude dotctl/config.yml (required for apply to work on fresh machines)
• Add install.sh: detects OS/arch, downloads binary from GitHub releases, optionally runs dotctl apply
• Update README with Quick Start section covering both bootstrap methods
• Rewrite test/link_test.go with 10 real-filesystem idempotency tests
• Add test/apply_test.go with 5 tests covering dry-run, linking, and idempotency
• Fix pre-existing TestInitCommand failure (missing MemMapFs setup)