feat(ui): a Claude-Desktop-style home for every workspace

/workspace?lane=<id> opens on the workspace's home (WorkspaceHome): a prompt
box whose Start opens a new session in the first free pane slot, Recents (a
live session attaches its pane, any other opens its transcript, nothing
resumes), and Instructions / Memory / Context / Folder. Instructions live in
lanes.instructions, extra folders and uploaded files in the new lane_sources
table (copies under <data dir>/workspace-context/<lane id>/); spawnRun reads
them from the lane's rows and hands Claude --add-dir + an appended system
prompt file, Codex --add-dir + developer_instructions. Memory is read-only.

Every dashboard pane carries AGENTHUB_RUN_ID; hooks forward it and the run
records which session the pane holds (a Claude pane follows /clear and
/resume but not a nested claude -p), so the sidebar and notifications open
the right pane (&run=). Settings loses its Projects group; Worktrees now
decides which worktrees Add folder suggests.

server/db.js

@@ -721,6 +721,30 @@ try {
   db.prepare("ALTER TABLE lanes ADD COLUMN needs_action_session TEXT").run();
 }
 
+// Migrate: a workspace's own context, as on a Claude Desktop project page
+// (server/lib/lane-sources.js). `instructions` is appended to every pane's
+// system prompt; `lane_sources` holds extra folders (the user's, never
+// deleted) and uploaded context files (AgentHub's own copies under
+// <data dir>/workspace-context/<lane id>/). Both additive.
+try {
+  db.prepare("SELECT instructions FROM lanes LIMIT 1").get();
+} catch {
+  db.prepare("ALTER TABLE lanes ADD COLUMN instructions TEXT").run();
+}
+db.exec(`
+  CREATE TABLE IF NOT EXISTS lane_sources (
+    id INTEGER PRIMARY KEY,
+    lane_id INTEGER NOT NULL,
+    kind TEXT NOT NULL CHECK (kind IN ('folder', 'file')),
+    path TEXT NOT NULL,
+    name TEXT NOT NULL,
+    size INTEGER,
+    added_at TEXT NOT NULL,
+    FOREIGN KEY (lane_id) REFERENCES lanes(id) ON DELETE CASCADE
+  );
+  CREATE INDEX IF NOT EXISTS idx_lane_sources_lane ON lane_sources(lane_id);
+`);
+
 // Seed lane_stage_events from the snapshots that predate it, so upgrading does
 // not read as "every pipeline has never run". Each lane's live `stages` blob
 // becomes one run; each archived feature becomes one more. That is all the