Prompt-template leftovers: non-word token warnings + descriptionRaw docs

[andrewklingelhofer]

Found during review of #367.

1. The unknown-token warning regex /\{\{(\w+)\}\}/ (src/prompt-template.ts:40) only matches word chars, so a typo like {{base-branch}} or {{pr.number}} is left intact silently — no warning. Consider warning on any {{...}}-shaped token that isn't recognized.
2. {{descriptionRaw}} (the new raw-description escape hatch) is undocumented — src/prompt-help.ts:42-43 lists variables but omits it (it also omits the pre-existing baseBranch/prUrl/prNumber/prBranch).