docs(release): establish stable-channel cadence + promotion process (#565)

Closes #565.

Beta has been the de-facto stable channel because the actual stable
lagged so far behind that new users following the README ended up
worse off than users who knew to switch to beta. The fix has two
parts: regular stable cuts (the PR #568 promotion is the first one)
and a written process so future cuts don't depend on memory.

This adds:

- RELEASING.md at the repo root — full operational doc with cadence
  target (4–6 weeks), promotion criteria (CI green + 7-day bug soak
  + upgrade-walk on real-shaped data + operator smoke), the actual
  beta→main mechanics including the conflict-resolution checklist we
  used in PR #568, hotfix backport path (with PR #412 as the worked
  example), and the project's versioning rules.
- CONTRIBUTING.md — replaces the four-line "Release Process" stub
  (which was wrong; it described a hand-rolled flow that release-please
  has handled for the last several releases) with a brief summary and
  a pointer to RELEASING.md.
- README.md — one-sentence addition to the existing "Release Channels"
  section pointing curious users at RELEASING.md.

No code change. CHANGELOG.md and version files are intentionally
untouched — release-please will catch this on the next regular cut.

README.md

@@ -97,7 +97,7 @@ Note on Docker file permissions
 
 ## 🔄 Release Channels
 
-PicPeak offers two release channels for different needs:
+PicPeak offers two release channels for different needs. Stable promotions are cut from a known-good beta point every 4–6 weeks — see [RELEASING.md](RELEASING.md) for the maintainer's promotion criteria and cadence policy.
 
 ### Stable Channel (Recommended)
 - Production-ready releases