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

.pi/prompts/is.md

@@ -1,5 +1,6 @@
 ---
 description: Analyze GitHub issues (bugs or feature requests)
+argument-hint: "<issue>"
 ---
 Analyze GitHub issue(s): $ARGUMENTS
 

.pi/prompts/pr.md

@@ -1,5 +1,6 @@
 ---
 description: Review PRs from URLs with structured issue and code analysis
+argument-hint: "<PR-URL>"
 ---
 You are given one or more GitHub PR URLs: $@
 

.pi/prompts/wr.md

@@ -1,5 +1,6 @@
 ---
 description: Finish the current task end-to-end with changelog, commit, and push
+argument-hint: "[instructions]"
 ---
 Wrap it.