docs(coding-agent): update README intro, add packages/prompt-templates/themes docs

- Rewrote README intro to emphasize extensibility and pi packages
- Added Pi Packages section to README
- Created docs/packages.md covering install, sources, manifest, filtering
- Created docs/prompt-templates.md covering format and arguments
- Created docs/themes.md with overview linking to theme.md

packages/coding-agent/docs/themes.md

@@ -0,0 +1,46 @@
+> pi can create themes. Ask it to build one for your setup.
+
+# Themes
+
+Themes are JSON files that define colors for the TUI. You can select themes in `/settings` or via `settings.json`.
+
+## Locations
+
+Pi loads themes from:
+
+- Built-in: `dark`, `light`
+- Global: `~/.pi/agent/themes/*.json`
+- Project: `.pi/themes/*.json`
+- Packages: `themes/` directories or `pi.themes` entries in `package.json`
+- Settings: `themes` array with files or directories
+- CLI: `--theme <path>` (repeatable)
+
+Disable discovery with `--no-themes`.
+
+## Format
+
+Themes use this structure:
+
+```json
+{
+  "$schema": "https://raw.githubusercontent.com/badlogic/pi-mono/main/packages/coding-agent/theme-schema.json",
+  "name": "my-theme",
+  "vars": {
+    "accent": "#00aaff",
+    "muted": 242
+  },
+  "colors": {
+    "accent": "accent",
+    "muted": "muted",
+    "text": "",
+    "userMessageBg": "#2d2d30",
+    "toolSuccessBg": "#1e2e1e"
+  }
+}
+```
+
+- `name` is required and must be unique.
+- `vars` is optional. It lets you reuse colors.
+- `colors` must define all required tokens.
+
+See [theme.md](theme.md) for the full token list, color formats, and validation details.