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/coding-agent/CHANGELOG.md

@@ -270,6 +270,8 @@ await runtime.fork("entry-id");
 
 - Added label timestamps to the session tree with a `Shift+T` toggle in `/tree`, smart date formatting, and timestamp preservation through branching ([#2691](https://github.com/badlogic/pi-mono/pull/2691) by [@w-winter](https://github.com/w-winter))
 
+- Added `argument-hint` frontmatter field for prompt templates, displayed before the description in the autocomplete dropdown ([#2761](https://github.com/badlogic/pi-mono/issues/2761))
+
 ### Fixed
 
 - Fixed startup resource loading to reuse the initial `ResourceLoader` for the first runtime, so extensions are not loaded twice before session startup and `session_start` handlers still fire for singleton-style extensions ([#2766](https://github.com/badlogic/pi-mono/issues/2766))