feat(themes): add color derivation and themed app templates

Add script/omarchy-theme-color and expand omarchy-theme-set-templates to
derive a full palette from each theme's colors.toml, then render new
themed templates (shell, pi, vscode theme, gum, neovim, claude, hermes,
t3code) from it.

Adds backgrounds and palette updates for existing themes plus new themes
(last-horizon, lupine, solitude, tidewater-*, miasma, ...).

default/themed/t3code.json.tpl

@@ -0,0 +1,36 @@
+{
+  "name": "Omarchy",
+  "appearance": "{{ mode }}",
+  "canvas": "{{ background }}",
+  "accent": "{{ accent }}",
+  "colors": {
+    "chrome": "{{ background }}",
+    "toolbar": "{{ background }}",
+    "toolbarForeground": "{{ foreground }}",
+    "toolbarBorder": "{{ muted }}",
+
+    "text": "{{ foreground }}",
+    "border": "{{ muted }}",
+    "focus": "{{ accent }}",
+
+    "sidebar": "{{ dark_background }}",
+    "sidebarForeground": "{{ foreground }}",
+    "sidebarBorder": "{{ muted }}",
+    "sidebarRowHover": "{{ mix dark_background foreground 6% }}",
+    "sidebarRowActive": "{{ mix dark_background accent 18% }}",
+    "sidebarRowSelected": "{{ selection }}",
+
+    "error": "{{ mix red foreground 35% }}",
+    "warning": "{{ mix yellow foreground 35% }}",
+
+    "codeBackground": "{{ dark_background }}",
+    "codeForeground": "{{ foreground }}",
+
+    "terminalBackground": "{{ background }}",
+    "terminalForeground": "{{ foreground }}",
+    "terminalCursor": "{{ bright_foreground }}",
+    "terminalSelection": "{{ selection }}",
+    "terminalScrollbar": "{{ muted }}",
+    "terminalScrollbarHover": "{{ dark_foreground }}"
+  }
+}