CONTRIBUTING.md: document single-trunk + RC-tag release model

CONTRIBUTING.md

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+# Contributing
+
+`breadhelp` — Onboarding and help center for Bread OS.
+
+Part of the bread ecosystem; this repo follows the same branch/release
+workflow as every other ecosystem product.
+
+## Branches
+
+There is one long-lived branch: **`main`**. All day-to-day work lands here.
+Every push to `main` automatically builds and publishes a **dev-track**
+build (see Tracks below) — a real install you can test before cutting
+anything more formal.
+
+New work — features and bug fixes alike — goes on a short-lived branch:
+
+```
+feature/<short-name>
+fix/<issue-number-or-short-name>
+```
+
+Branch off `main`, open a PR/push back into `main` when ready. Short-lived
+branches get deleted on merge — they never accumulate the kind of drift a
+second long-lived branch does.
+
+## The release cycle
+
+There's no separate `beta` or release branch — "stable" and "beta" are both
+just **tags** on `main`, not branches that need to be kept in sync:
+
+1. Work accumulates on `main` via `feature/x` / `fix/x` branches. Each push
+   auto-publishes a dev build — install it with `bakery track set dev` and
+   `bakery update --all`, then fix anything broken with another push.
+2. When you want to stabilize before a real release, tag a release
+   candidate: `git tag vX.Y.Z-rc.1 && git push origin vX.Y.Z-rc.1` (push to
+   both remotes). That tag alone triggers a beta-track build —
+   "freezing" is just pausing pushes to `main` while you test it, not a
+   branch operation. Cut `-rc.2`, `-rc.3`, etc. for further fixes.
+3. Once an RC has gone without issues, tag the real release:
+   `git tag vX.Y.Z && git push origin vX.Y.Z` — that's what triggers the
+   signed stable release build.
+
+## Tracks, from a user's perspective
+
+```
+bakery track show              # what you're currently on (defaults to stable)
+bakery track set dev           # or beta, or stable
+bakery update --all            # pull the latest build on your current track
+```
+
+| Track  | What it is | Published from |
+|--------|-----------|-----------------|
+| `stable` | The last tagged release | a `vX.Y.Z` tag |
+| `beta` | Latest release candidate | a `vX.Y.Z-rc.N` tag |
+| `dev` | Bleeding edge | `main`, on every push |
+
+Dev versions are auto-computed (`X.Y.Z-dev.<timestamp>+<sha>`) from the
+latest published stable tag, so they always sort as newer than what you
+have installed — no manual version bumping needed. Beta versions are just
+the RC tag itself (already valid semver, already sorts below the real
+release it's a candidate for).
+
+## Local development
+
+```sh
+cargo build --release
+cargo test --release
+```
+
+## CI
+
+- `dev-release.yml` — triggered on push to `main`.
+- `rc-release.yml` — triggered on any `vX.Y.Z-rc.N` tag push.
+- `release.yml` — triggered on any other `v*` tag push, cuts the actual
+  stable release.
+
+All CI runs on a self-hosted runner; nothing runs automatically on plain
+commits or PRs beyond the track builds above. See
+[bread-ecosystem's docs/release-channels.md](https://git.breadway.dev/Breadway/bread-ecosystem/src/branch/main/docs/release-channels.md)
+for the full policy, including how a new product gets wired onto these tracks.
+
+## Questions
+
+Open an issue on this repo's Forgejo tracker.