feat(coding-agent,tui): support argument-hint frontmatter in prompt templates (#2780)

* 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

packages/tui/CHANGELOG.md

@@ -62,6 +62,10 @@
 - Fixed slash-command argument autocomplete to await async `getArgumentCompletions()` results and ignore invalid return values, preventing crashes when extension commands provide asynchronous completions ([#2719](https://github.com/badlogic/pi-mono/issues/2719))
 - Fixed non-capturing overlay padding from inflating scrollback and corrupting the viewport on terminal widen ([#2758](https://github.com/badlogic/pi-mono/pull/2758) by [@dotBeeps](https://github.com/dotBeeps))
 
+### Added
+
+- Added `argumentHint` to `SlashCommand` interface, displayed before the description in the autocomplete dropdown ([#2761](https://github.com/badlogic/pi-mono/issues/2761))
+
 ## [0.64.0] - 2026-03-29
 
 ### Fixed