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/preset/preset.go

@@ -300,14 +300,6 @@ func ensureDefaults(base string) error {
     "^\\s*>_"
   ]
 }
-`,
-		},
-		{
-			"presets/processes/shell.json",
-			`{
-  "name": "shell",
-  "argv": ["__SHELL__"]
-}
 `,
 		},
 	}
@@ -319,15 +311,7 @@ func ensureDefaults(base string) error {
 		if err := os.MkdirAll(filepath.Dir(full), 0o700); err != nil {
 			return err
 		}
-		body := d.body
-		if strings.Contains(body, "__SHELL__") {
-			shell := os.Getenv("SHELL")
-			if shell == "" {
-				shell = "/bin/sh"
-			}
-			body = strings.ReplaceAll(body, "__SHELL__", shell)
-		}
-		if err := os.WriteFile(full, []byte(body), 0o600); err != nil {
+		if err := os.WriteFile(full, []byte(d.body), 0o600); err != nil {
 			return err
 		}
 	}