Adopt branch model: develop integration branch + per-merge upstream-sync branches

[CodeSculptor]

Context

The release-process review recommended a branch model that separates upstream sync, feature work, and bug fixes so releases stay coherent (no "archaeological software" where four kinds of change pile into one release).

Current flow: worktree branch → push directly to master. There is no integration branch, and the upstream auto-merge bot pulls upstream master (mixing stable + beta heads).

Proposed model

main                    # always releasable-ish, no direct agent churn
develop                 # integration branch for phone-UI work
upstream-sync/st<ver>   # one branch per upstream merge, upstream-only changes
feature/<name>          # small PRs into develop
fix/<name>

Release flow: upstream-sync/* → build + smoke test → develop; feature/* → PR → develop; develop → release/vX.Y.Z → checklist → tag → release → back-merge to main.

Also (separate upstream tracks)

Split the sync source so public beta/stable builds default to SmartTube stable tags, and only alpha/internal builds track SmartTube beta/head. Gate C correctly landed on 31.94 stable, but by convention, not by mechanism.

Acceptance criteria

• develop branch exists and is the default integration target
• Upstream merges land on upstream-sync/* branches first, not straight to master
• Auto-merge bot config documents stable-vs-beta track policy
• RELEASE_PROCESS.md updated to describe the model

Notes

Acknowledged trade-off: this is heavier than the current single-maintainer flow. May be deferred until there are external contributors — but tracking it so the decision is explicit, not forgotten.

[CodeSculptor]

Deferring for now — keeping the current incremental-to-master model.

For a solo developer + coding-agent workflow the develop/feature/upstream-sync branch model adds overhead without a clear payoff right now: each shippable piece is verified on device and lands on master directly, and upstream syncs already get their own upstream-sync/* branch. Revisit when contributor volume (multiple parallel contributors / PRs) justifies the integration branch.