aitools: register Claude's official marketplace before install

[rclarey]

Changes

No longer treat claude-plugins-official marketplace as built-in and manually install it before installing the databricks plugin.

This PR splits the two things an empty Source used to conflate: Source now only names the repo to marketplace add, and a new Shared flag marks a marketplace we don't own. Claude gets a real Source so the marketplace is added before the update+install, plus Shared=true so uninstall never de-registers the shared marketplace. The uninstall message is updated to match (it no longer claims to remove a Shared marketplace).

Why

The claude-plugins-official marketplace was not reliably registered locally (e.g. when ucode claude first installs Claude). The installer treated it as a built-in marketplace (empty Source) that must never be added, so the pre-install plugin marketplace update had nothing to refresh and the databricks plugin install failed with "not found in marketplace".

Tests

updated unit tests + manually verified via ucode claude (reliable repro)

[github-actions]

Waiting for approval

Based on git history, these people are best suited to review:

• @simonfaltum -- recent work in libs/aitools/installer/, libs/aitools/agents/

Eligible reviewers: @Divyansh-db, @MarioCadenas, @Shridhad, @atilafassina, @calvarjorge, @chrisst, @ditadi, @fjakobs, @hectorcast-db, @igrekun, @keugenek, @lennartkats-db, @mihaimitrea-db, @parthban-db, @pffigueiredo, @pkosiec, @rauchy, @renaudhartert-db, @tanmay-db, @tejaskochar-db

_{Suggestions based on git history. See OWNERS for ownership rules.}