refactor(settings): scope the settings project selector to settings

Picking a project in Settings called setActiveProject, which relocates
the chat, the session list, the file tree and the Git surface. Reading
another project's MCP servers or agents moved the user's whole app.

It had to, because the configuration stores resolved the directory
themselves from the active project and held one flat list. Each of them
now takes an explicit directory — omitted still means the active project,
so every caller outside Settings is unchanged — and keys loaded data by
directory next to a flat mirror of the active project. Chat, autocompletes
and pickers keep reading that mirror; a load for another directory writes
only the map. A failed load restores that directory's previous list.

Settings resolves its own directory through useSettingsDirectory, backed
by a session-local settingsProjectPath that follows the active project
until the user picks something else.

CHANGELOG.md

@@ -9,6 +9,7 @@ All notable changes to this project will be documented in this file.
 - Desktop/Remote instances: a managed remote server can now also be published to the remote machine's own network, so other devices there reach it without the SSH tunnel. It requires a UI password, and stays private to the tunnel otherwise.
 - Desktop/Remote instances: disconnecting from a connection set to not keep the server running now actually stops that remote server.
 - Chat: in a chat without a project, the work status card again steps aside when the context panel is open, instead of sitting next to it.
+- **Settings:** the project selector on Providers, Agents, MCP, Commands and Skills now only changes what those pages show. It used to switch the whole app, so opening another project's configuration moved your chat, session list and file tree with it.
 - Settings/Providers: the provider you select no longer jumps to a different one on its own. Changing the chat's model or agent, and background provider refreshes, used to move the settings selection with them.
 - **Chat sessions:** start chats without choosing a project. They live in their own Chats section, rather than inheriting a project's repository and worktree context.
 - **Skills catalog:** browse curated GitHub skill collections in a card-based catalog with cross-source search, skill counts, stars, recent updates, and links back to each skill's repository.