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/modes/interactive/interactive-mode.ts

@@ -432,6 +432,7 @@ export class InteractiveMode {
 		const templateCommands: SlashCommand[] = this.session.promptTemplates.map((cmd) => ({
 			name: cmd.name,
 			description: this.prefixAutocompleteDescription(cmd.description, cmd.sourceInfo),
+			...(cmd.argumentHint && { argumentHint: cmd.argumentHint }),
 		}));
 
 		// Convert extension commands to SlashCommand format