* feat(coding-agent,tui): support argument-hint frontmatter in prompt templates
Parse argument-hint from prompt template frontmatter and display it
in the autocomplete dropdown description, matching Claude Code's
convention for custom commands.
Frontmatter format:
---
description: Code review
argument-hint: "[file | #PR | PR-URL]"
---
The hint renders in the description column of the autocomplete list:
review [file | #PR | PR-URL] — Code review
Closes#2761
* docs(coding-agent,tui): add argument-hint documentation, tests, and built-in hints
- Document argument-hint frontmatter in prompt-templates.md with <required>/[optional] convention
- Add argument-hint to built-in prompts: pr, is, wr
- Expand tests: required/optional hints, missing hints, empty hints, special characters
- Add changelog entries for coding-agent and tui
- 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
