persist last-opened project and wire welcome open dialog

ProjectManager.open() now saves app.lastProject to user settings.
Boot calls openLast() instead of opening cwd — first launch shows
the welcome screen, subsequent launches restore the last project
directly into Claude. The welcome "Open project" button shows a
modal path-entry dialog that validates against git and activates
the Claude tab on success. Removed the dead "New Claude session"
button since secondary sessions are spawned via command palette.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -65,6 +65,11 @@ heading, and (b) bumping `project.yaml` `version:` in the same commit.
 - Graph view in context panel — lists files with inbound/outbound
   link counts from `pql search --connections` (T-039).
 
+- Last-opened project persisted to user settings and restored on
+  boot. First launch shows the welcome screen with a path-entry
+  dialog; subsequent launches re-open the last project directly
+  into Claude.
+
 ### Changed
 
 - Workspace renders Claude as the always-visible primary surface