Enforce trunk-based branching policy (no direct commits to protected branches)

[wimaan3]

Problem

borromeanRings already follows trunk-based development in practice — short-lived feature branches → PR → gated merge (merge.sh --delete-branch) → delete — so main only ever receives PR merges (0 direct commits, verified via the GitHub activity log). But the policy is implicit, not enforced: nothing actually prevents a direct commit/push to main. Governed projects inherit the same gap, and at a glance they look "main-only" because feature branches are auto-deleted after merge.

Decision: standardize on trunk-based (chosen over GitFlow — for an always-green gated project a long-lived develop branch adds ceremony without payoff) and make the branching policy a declared, enforced borromeanRings capability.

Scope

• Declare protected branches in the spine (e.g. [branching] protected = ["main"], default ["main"]).
• Preventive guard (PreToolUse): block git commit when HEAD is on a protected branch, and block direct git push to a protected branch — with a clear message ("create a feature branch; land via PR + gate"). Pure logic unit-tested; fail-open on guard error (server-side protection is the backstop).
• Works for governed projects (reads policy from the project's borromeanrings.toml, reference-mode safe).
• Document the model (trunk-based) and how it composes with the gated merge.

Out of scope / complementary

• Server-side branch protection on main (require the gate check, block direct pushes) is tracked separately in Branch protection on main (require the gate) #60 — this issue is the local, harness-enforced, substrate-neutral layer; Branch protection on main (require the gate) #60 is the GitHub-side enforcement. Both together = defense in depth.

Acceptance criteria

• [branching].protected declared in the spine (default ["main"]); empty ⇒ enforcement off (opt-in, consistent with [git]/[layout]).
• Guard denies a commit made while on a protected branch (with a fix hint).
• Guard denies a direct git push to a protected branch.
• Unit tests for the decision logic; verified in a governed temp project.
• ADR; docs note the trunk-based model.

Relates to: #60 (server-side branch protection), and the existing PreToolUse guard (.claude/hooks/pre_bash_guard.sh) + gated merge flow (merge.sh).