feat(theme): add theme randomizer and save-to-disk

Theme::random() conjures a fresh procedural vestment — a coherent HSV
palette (dark tinted surface, one bright accent, legible ink), a random
sigil, and a generated arcane name. Bound to Ctrl+Alt+P and `/theme random`.

Theme::save() persists the vestment you're wearing to themes/<slug>.toml
(via `/theme save [name]`), so a roll you like can be re-donned later with
`/theme <name>`. Theme now derives Serialize and slugify() sanitizes the
filename. Help text and the /theme usage line advertise both verbs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hh/src/ui.rs

@@ -146,7 +146,7 @@ fn help_lines(theme: &Theme) -> Vec<Line<'static>> {
     // List whatever vestments are actually installed, so new themes show up here
     // automatically (church · neon · crypt · blush · matrix · wraith · …).
     let theme_help = format!(
-        "change vestments live: {}",
+        "vestments: {} · random · save [name]",
         Theme::available().join(" · ")
     );
     vec![
@@ -209,6 +209,14 @@ fn help_lines(theme: &Theme) -> Vec<Line<'static>> {
             "Ctrl-R  (when closed)",
             "reconnect to the house after a drop / AFK",
         ),
+        kv(
+            "Ctrl+Alt+P  ·  /theme random",
+            "conjure a random vestment (new palette + sigil)",
+        ),
+        kv(
+            "/theme save [name]",
+            "keep the vestment you're wearing for reuse",
+        ),
         kv("Ctrl-C  ·  Ctrl-Q", "quit hack-house"),
         Line::from(""),
         head("ROSTER GLYPHS"),