Rename Kill to Close, add New Terminal palette entry, clean up exited terminals

- Palette's per-child "Kill <name>" action is now labelled "Close <name>"
  (action kind unchanged; still SIGTERM). Matches the existing "Close
  agent: …" context entry and reads less violent for a graceful term.
- New "New Terminal" palette entry spawns a bare interactive $SHELL pane
  via LaunchTerminal (kind=terminal). Replaces the default "shell"
  process preset that was seeded on first run.
- Exited KindTerminal entries are now dropped from the session in
  reapChild — terminals have no restart path, so leaving them behind as
  greyed rows in the Processes sidebar was just clutter. processList
  also filters defensively.

internal/app/session.go

@@ -403,6 +403,23 @@ func (s *Session) reapChild(c *Child, runID uint64) {
 	if !c.restarting.Load() {
 		c.cleanupOwnedPaths()
 	}
+	// Terminals are ephemeral: unlike command entries (kept around for
+	// restart_process) and agents (which the user clears via close_process
+	// once they're done with the corpse), an exited terminal has nothing
+	// useful left to do. Drop it from the session so it disappears from
+	// the Processes sidebar / switch list immediately.
+	if c.Kind == KindTerminal && !c.restarting.Load() {
+		c.teardownPTY()
+		s.mu.Lock()
+		delete(s.children, c.ID)
+		for i, oid := range s.order {
+			if oid == c.ID {
+				s.order = append(s.order[:i], s.order[i+1:]...)
+				break
+			}
+		}
+		s.mu.Unlock()
+	}
 }
 
 // killDescendantsOf terminates every still-live direct child of