docs(contributing): name the merge rules main now enforces

[somework]

What this changes

CONTRIBUTING gains a short Opening a pull request section: main takes no direct push, all eight CI checks are required, the branch must be current at merge time, no approval is required, and history stays linear.

Why

Branch protection was just enabled. A rule the contributing guide does not name is a rule people meet as a surprise at merge time.

This pull request is also the empirical check that the gate works — it is the first one the protection has seen.

How it was verified

• go test -race ./... passes
• go vet ./... && go vet -tags=live ./... && gofmt -l . is clean
• go run honnef.co/go/tools/cmd/staticcheck@v0.7.0 ./... is clean
• go run golang.org/x/vuln/cmd/govulncheck@latest ./... reports nothing
• TestPublishedDocsNameOnlyRealCommandsAndFlags covers the edited file and passes

Invariants

Touches invariants: none — documentation only.

Dependencies

• go.mod is unchanged

Summary by CodeRabbit

• Documentation
  • Added guidance for opening pull requests, including required CI checks, branch synchronization, review expectations, and linear-history requirements.
  • Clarified that direct pushes to main are not permitted.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 2070d891-12aa-48a3-bc9a-8d0734c0d4fa

📥 Commits

Reviewing files that changed from the base of the PR and between cc6ad88 and b9caa05.

📒 Files selected for processing (1)

• CONTRIBUTING.md

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📝 Walkthrough

Walkthrough

The contribution guide adds an “Opening a pull request” section covering protected changes to main, CI checks, branch currency, approvals, and linear history.

Changes

Pull Request Guidelines

Layer / File(s)	Summary
Document pull request requirements CONTRIBUTING.md	Adds rules requiring pull requests for changes to main, successful CI checks, an up-to-date branch, no special approval requirement, and squashed or rebased history.

Estimated code review effort: 1 (Trivial) | ~2 minutes

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/pr-flow

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