Add a Shared Workspaces view

[EhabY]

Add a Shared Workspaces view to the Coder sidebar so users can see workspaces that other people have shared with them, alongside their own.

Tracked by #991.

What it does

• New Shared Workspaces tree view in the Coder activity bar, shown only when authenticated (when: coder.authenticated), sitting beneath My Workspaces.
• Lists workspaces returned by the shared:true query, with the owner shown (owner / workspace) since these belong to other users.
• Filters out workspaces the current user owns and shared outward, so the view shows only what was shared with them.
• Has its own search/filter command (coder.searchSharedWorkspaces) and refreshes on the same interval and sign-in/out events as the other views.

Implementation notes

• WorkspaceProvider is now query-driven via a per-view config (showOwner / showMetadata / excludeOwn), so My / All / Shared share one implementation. WorkspaceQuery.Shared = "shared:true" was added.
• Workspace refresh keys off a shared session state (WorkspaceSessionState) exposed by DeploymentManager, with a dedicated SessionStore owning sign-in/out transitions and revision tracking. This drives clear-and-refetch when the user signs in or out, and prevents stale results from races during deployment switches.
• View registered as sharedWorkspaces in package.json and wired in extension.ts.

Verification

pnpm typecheck, pnpm lint, and the full unit suite (1854 passed / 1 skipped) all green.