Implement `func update` command

Spec lives at `docs/proposed/func-update.spec.md` (on the `docs` branch). Implement the command
so users can update an installed v5 CLI to the latest version without
re-running the installer script.

## Definition of Done

- [ ] `func update` command implemented per the existing spec.
- [ ] Works across the installer's supported channels (script-based
install for Windows, macOS, Linux).
- [ ] Surfaces "no update available" cleanly when already current.
- [ ] Honours `--yes` to skip confirmation (required when non-interactive).
- [ ] Tests cover happy path + no-update path + failure path.
- [ ] Help text + docs updated.

## PRs

| PR | Status | What it delivers |
|----|--------|-----------------|
| #5297 | ✅ Merged | `UpdateCommand` scaffold — options (`--prerelease`, `--version`, `--yes`), help text, Parser registration |
| #5298 | ✅ Merged | `IReleaseFeed` + `CdnReleaseFeed` — fetches version manifest from CDN, resolves latest stable/preview |
| #5428 | Draft | `ICliUpdater` + `CliUpdater` — download → extract → backup → swap → verify → rollback pipeline |
| #5458 | Draft | Wire `UpdateCommand` handler — install-method detection (npm/brew/choco/winget), version compare, confirmation prompt, progress reporting, DI registration. Completes the feature. |