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/src/core/prompt-templates.ts

@@ -11,6 +11,7 @@ import { createSyntheticSourceInfo, type SourceInfo } from "./source-info.js";
 export interface PromptTemplate {
 	name: string;
 	description: string;
+	argumentHint?: string;
 	content: string;
 	sourceInfo: SourceInfo;
 	filePath: string; // Absolute path to the template file
@@ -121,6 +122,7 @@ function loadTemplateFromFile(filePath: string, sourceInfo: SourceInfo): PromptT
 		return {
 			name,
 			description,
+			...(frontmatter["argument-hint"] && { argumentHint: frontmatter["argument-hint"] }),
 			content: body,
 			sourceInfo,
 			filePath,