chore(branching): adopt trunk-on-main default and retain zi integration

[ss-o]

Objective

Implement the 2026-08-28 maintainer decision to make protected trunk-on-main
the organization default and retain next -> main only for z-shell/zi.

Decision

Accepted by maintainer ss-o on 2026-08-28:

• src, wiki, zsh-lint, and zsh-eza migrate to trunk-on-main.
• zi retains next as its integration branch because main is a directly
  consumed stable ref and the repository has a distinct promotion qualification
  suite.
• Persistent next -> main promotion uses a merge commit. The destination
  branch does not require linear history, and routine squash reconciliation is
  retired.
• Deployment and publication controls remain separate from code integration:
  class 1 uses reviewed main plus deployment controls; class 2 publishes from
  annotated tags; class 3 uses reviewed refs according to its assigned model.

Required deliverables

• Supersede ADRs 0008 and 0016 with an accepted replacement decision.
• Reconcile branch protection, release, dependency, onboarding, and new-repo
  runbooks plus routed instructions and validators.
• Promote all outstanding wiki and zsh-lint integration work before
  retiring their persistent branches.
• Migrate src, wiki, zsh-lint, and zsh-eza workflows, dependency
  targets, documentation, rulesets, and repository settings to main.
• Correct zi promotion mechanics and rulesets while retaining next.
• Update the private workspace catalog and regenerate its instruction
  composite.
• Prove every retired next branch is represented in main, has no open
  dependent pull requests, then remove its ruleset and branch.
• Run repository-specific checks and a final live settings audit.

Instruction impact review

1. Classification: shared branching policy, scoped operational guidance, and
   repository enforcement.
2. Consumers: maintainers and all supported agent runtimes across the public
   organization policy, affected repositories, and private control workspace.
3. Canonical owner: a new ADR in z-shell/.github, with operational detail in
   the existing branch, release, dependency, onboarding, and bootstrap runbooks.
4. Duplicate or contradictory surfaces: ADRs 0008 and 0016, ADR 0013 rationale,
   branch-protection and release reconciliation procedures, child AGENTS.md,
   workflow contracts, dependency targets, and the wiki contributor guide.
5. Manifest routing: no new route is expected; existing branch-protection,
   workflow, documentation, and instruction-change routes remain the owners.
6. Runtime delivery: the public AGENTS.md reference and private generated
   composite will be updated and validated without relying on optional hooks.
7. Validation: public policy validators, private generation checks,
   repository-specific tests, workflow parsing/actionlint, and live ruleset and
   branch audits.

Triage

• Item type: Chore
• Impact: High
• Effort: XL
• Priority: P1
• Workstream: Administrative