fix(pulls+issues): first-PR dead ends die — forms keep your input, templates render, single-branch guidance #5547

Queue item #1 from the flow audit — three findings with one shared root, the new-PR form:

1. Forms no longer wipe what you typed. Every POST validation failure used to redirect to ?error= and land on a blank form — a one-word mistake cost the whole description. The form is now a shared renderer and every error path re-renders with the typed title, body, and branch selections preserved.
2. Single-branch repos get guidance, not a broken form. branches.filter(b => b !== base) rendered an empty head dropdown that submitted into a cryptic error — a hard dead end on the fresh-repo → first-PR journey. Now: "a PR compares two branches, and this repository has only main" plus the exact commands, and a way back.
3. PR and issue templates finally do something. loadPrTemplate/loadIssueTemplate were called and their results discarded for as long as the forms have existed — a team committing .github/PULL_REQUEST_TEMPLATE.md saw nothing, with no error. Both now prefill their body TextAreas (PR side only when the user hasn't typed their own body).

?error=/?head= on GET kept for old links. Pinned by first-pr-form-honesty.test.ts (5 tests). Typecheck clean; full local sweep of pull-matching suites green.