Add update-profile backup and removal support

[codeforester]

Problem

basectl update-profile mutates shell startup files by writing Base-managed sections, but the command does not surface an automatic backup of pre-edit files or a --remove path to remove Base-managed sections. Shell startup mutation is a high-trust part of adoption.

Evidence

• update-profile --help lists --defaults, --no-defaults, and --dry-run, but no removal option.
• The update path writes managed sections into ~/.bash_profile, ~/.bashrc, ~/.zprofile, and ~/.zshrc.

Desired outcome

Give users a reversible profile update path that preserves pre-edit files before mutation and can remove Base-managed sections without hand-editing dotfiles.

Scope

• Add automatic backup behavior for files that will be changed.
• Add an explicit remove path for Base-managed profile sections.
• Keep dry-run behavior accurate.
• Do not remove user-authored content outside Base-managed markers.

Acceptance criteria

• basectl update-profile creates timestamped backups before changing existing dotfiles.
• basectl update-profile --remove removes only Base-managed sections.
• --dry-run reports planned backups/removals without writing.
• BATS cover update, remove, backup, and dry-run behavior.
• README or command reference documents the reversible path.