docs: add upstream-PR workflow + lnbits branch-model reference

Two new docs under docs/:

- upstream-prs.md — how to send a PR upstream using the
  ~/dev/upstream-prs/ worktree flow. Opens with a "Primer" section
  for anyone new to forks / pull requests (the three remote roles —
  upstream / github-fork / origin — explained as a table), then the
  per-project prerequisites, the prb/prc helper-driven workflow, and
  a "Common pitfalls" section.

- lnbits-upstream-flow.md — reference for how lnbits/lnbits actually
  moves: the dev/main branch split, the squash-merge PR convention,
  the non-FF release merge from dev into main, and how to read the
  history with --first-parent. Adapted from internal aiolabs notes
  with the fork-versioning specifics stripped; closing section is
  generic guidance for anyone maintaining their own long-lived fork.

README links both from a new "Further reading" section. The prior
"Contributing" section is retitled "Contributing to this scaffold"
to avoid colliding with the new upstream-PR doc on the term.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -192,7 +192,18 @@ lb dev             # cd ~/dev/lnbits/dev
 lb fix-issue-123   # cd ~/dev/lnbits/fix-issue-123
 ```
 
-## Contributing
+## Further reading
+
+- [`docs/remotes.md`](docs/remotes.md) — the three remote-topology
+  patterns (upstream-only / github-fork / multi-remote).
+- [`docs/upstream-prs.md`](docs/upstream-prs.md) — how to send a PR
+  upstream using the `~/dev/upstream-prs/` worktree flow. Includes
+  a primer for anyone new to forks / pull requests.
+- [`docs/lnbits-upstream-flow.md`](docs/lnbits-upstream-flow.md) —
+  reference for how `lnbits/lnbits` itself moves: `dev` / `main`
+  branch model, squash-merge convention, release tagging.
+
+## Contributing to this scaffold
 
 Iteration happens under the sandboxed-claude policy from omnixy's
 `scripts/sandbox-claude.sh` — a per-launch `.claude/settings.json`