feat: import GitHub issues and PRs from QuickCreate

[Codename-11]

Summary

• Adds a QuickCreate import path for pasted GitHub issue/PR URLs and owner/repo#number shorthand.
• Reuses the existing GitHub mapping/linkability + preview flow so the overlay shows the resolved issue/PR before importing.
• Extends github.importIssue/backend sync to accept canonical URLs and PR resources, including issue-number shorthands that resolve to pull requests.
• Updates the themed Combobox ARIA contract and stale E2E status-picker interactions uncovered by CI.

Verification

• pnpm typecheck
• pnpm lint (pre-existing native-select warnings only)
• pnpm exec vitest run tests/unit/quick-create-modes.test.ts tests/unit/github-support.test.ts --reporter=dot
• set -a; source /home/bailey/forge/.env; set +a; env -u OPENAI_API_KEY pnpm test -- --reporter=dot (1032 passed / 1 skipped)
• pnpm exec playwright test tests/e2e/issue-flow.spec.ts tests/e2e/mobile-smoke.spec.ts --workers=1 --reporter=line (8 passed)

Notes

• First full-suite attempt in the isolated worktree failed because the worktree had no .env/DATABASE_URL; reran with the repo env loaded and OPENAI_API_KEY unset.
• Initial CI Playwright failure exposed stale status-picker tests after the app moved issue status/priority to Combobox; fixed and force-pushed.

Closes AXI-90