dotnetup: Walkthrough UX improvements (tracking issue)

## Summary

This is the tracking issue for walkthrough UX improvements discussed in our design review meeting and in the PR feedback threads ([dsplaisted's initial review](https://github.com/dotnet/sdk/pull/53464/#issuecomment-4079682125), [follow-up discussion](https://github.com/dotnet/sdk/pull/53464/#issuecomment-4083888559)).

The goal is to simplify the first-run experience, reduce the number of prompts a user must answer, and make the walkthrough flow more intuitive — taking inspiration from tools like rustup while keeping the output minimal and readable.

## Sub-issues

### Core flow changes
- [ ] #53837 — Migrate walkthrough to summary-based approach
- [ ] #53838 — Migrate mode selector to two sequential y/n prompts
- [ ] #53839 — Add `dotnetup migrate` command for system-to-local install conversion
- [ ] #53840 — Consider renaming `walkthrough` command to `init` or alternative

### Bug fixes
- [ ] #53841 — Duplicate installs shown in system install migration list

### Polish
- [ ] #53842 — Improve walkthrough subtitle text
- [ ] #53843 — Add scroll indicator hint to channel selector

## Context

The [initial PR review](https://github.com/dotnet/sdk/pull/53464/#issuecomment-4079682125) identified several areas for improvement:
- The walkthrough should be skipped in non-interactive mode and when an install path is specified.
- Mode selection should use simpler yes/no questions instead of a 3-option selector.
- The channel prompt should be consistent between first-run and subsequent runs.
- Invalid inputs should be validated early before starting downloads.

The [follow-up discussion](https://github.com/dotnet/sdk/pull/53464/#issuecomment-4083888559) explored trade-offs around mode selection UX, channel prompt visibility, and the broader walkthrough structure. These sub-issues capture the agreed-upon direction from that conversation and subsequent design meetings.