feat: add confirmation prompt and --dry-run to skills install

[BRO3886]

Summary

Add a confirmation step and --dry-run flag to rem skills install, matching the pattern established in ical v0.8.0. Currently rem installs skill files immediately without any disclosure or confirmation.

What ical v0.8.0 does

1. Confirmation prompt — before writing any files, shows:

   • What the skill files are (documentation that teaches AI agents how to use the CLI)
   • That they contain the same content as the website docs
   • Exact paths that will be created
   • A Proceed with installation? confirm dialog
   • Only shown in interactive mode (TTY) — non-interactive/CI invocations skip it so scripts and agents can install without blocking

2. --dry-run flag — lists the exact files that would be written per target without actually writing anything

What rem currently does

rem skills install writes files immediately after target selection. No disclosure about what the files are, no confirmation, no dry-run option.

Proposed changes

1. Add --dry-run flag to rem skills install
2. Add a confirmInstall() step (interactive only) showing:
   • Brief explanation of what skill files are
   • Paths that will be created
   • Confirm dialog before proceeding
3. Non-interactive invocations (piped stdin, --agent in CI) skip the confirm intentionally

Reference

• ical implementation: cmd/ical/commands/skills.go — confirmInstall() and printDryRun() functions
• ical v0.8.0 release notes mention: "Skills install now shows a confirmation prompt before writing files, with --dry-run to preview"