Plugin install & marketplace management need interactive selection / autocomplete (currently full names must be memorized and typed manually)

## Summary
Plugin and marketplace management is currently fully manual: the user must memorize and type the full `owner/repo` (and for marketplaces, the marketplace name + plugin name) with no autocomplete, no interactive picker, and no list-based selection — even when the CLI clearly already has the relevant list available.

## Affected surfaces
- `copilot plugin install ` — no tab-complete, no fuzzy picker
- `copilot plugin marketplace install ` — same
- `copilot plugin marketplace browse ` — lists plugins but doesn't let you select one to install from the list
- `copilot plugin enable/disable` (see #2714, #3452) would have the same UX problem

## Expected
When invoked without arguments, these commands should drop into an interactive picker (similar to existing model/session pickers) backed by:
- For `install`: the marketplace catalog the CLI already has cached
- For `enable/disable`: the list of currently installed plugins
- For `browse ` → `install`: the plugin list already on screen — let the user arrow + Enter

## Related
- #1545 — "Have simpler plugin marketplace browse feature and install" (similar spirit)
- #2714, #3452 — plugin enable/disable (same UX class)
- #3360, #2077, #3376 — specific marketplace bugs (not UX)

## Why this matters
The current flow assumes the user has the full plugin coordinate memorized, which is fine for power users but defeats the discoverability point of having a marketplace at all.

Start with the plugin install, marketplace install, marketplace browse, and plugin enable/disable command entry points, then compare them with the existing model/session pickers. Trace how the CLI obtains marketplace and installed-plugin lists. Done means the specified commands support interactive selection from their available lists without requiring users to memorize full names.

Scritto dal modello di indicizzazione a partire dal testo della issue.