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/prompt-templates.md

@@ -0,0 +1,57 @@
+> pi can create prompt templates. Ask it to build one for your workflow.
+
+# Prompt Templates
+
+Prompt templates are Markdown snippets that expand into full prompts when you type `/name` in the editor.
+
+## Locations
+
+Pi loads prompt templates from:
+
+- Global: `~/.pi/agent/prompts/*.md`
+- Project: `.pi/prompts/*.md`
+- Packages: `prompts/` directories or `pi.prompts` entries in `package.json`
+- Settings: `prompts` array with files or directories
+- CLI: `--prompt-template <path>` (repeatable)
+
+Disable discovery with `--no-prompt-templates`.
+
+## Format
+
+```markdown
+---
+description: Review staged git changes
+---
+Review the staged changes (`git diff --cached`). Focus on:
+- Bugs and logic errors
+- Security issues
+- Error handling gaps
+```
+
+- The filename becomes the command name. `review.md` becomes `/review`.
+- `description` is optional. If missing, the first non-empty line is used.
+
+## Arguments
+
+Templates support positional arguments and simple slicing:
+
+- `$1`, `$2`, ... positional args
+- `$@` or `$ARGUMENTS` for all args joined
+- `${@:N}` for args from the Nth position (1-indexed)
+- `${@:N:L}` for `L` args starting at N
+
+Example:
+
+```markdown
+---
+description: Create a component
+---
+Create a React component named $1 with features: $@
+```
+
+Usage: `/component Button "onClick handler" "disabled support"`
+
+## Loading Rules
+
+- Template discovery in `prompts/` is non-recursive.
+- If you want templates in subdirectories, add them explicitly via `prompts` settings or a package manifest.