Docs: Require purpose prefixes for PRs and commits

[brandonpayton]

Why

PR titles and commit subjects are often the first—and sometimes only—context a developer sees while scanning CI, release history, or a long pull-request list. A mechanically worded title can make it hard for a junior developer to tell whether a change affects the kernel, POSIX behavior, Homebrew, CI, or documentation.

This change gives every title a short routing hint while preserving the existing requirement to explain the actual purpose of the work.

What changed

• Require the Area: Purpose shape for PR titles and commit subjects.
• Document common prefixes such as Homebrew:, Kernel:, POSIX:, CI:, and Docs:.
• Explain how to choose one primary prefix for cross-cutting work.
• Require squash commits to keep the PR’s semantic prefix so Git history remains easy to scan.

Contract scope

This is agent and contributor guidance only. It does not change runtime behavior, ABI, package artifacts, CI enforcement, Node.js behavior, or browser behavior.

Validation

• git diff --check origin/main...HEAD
• Reviewed the rendered Markdown structure and examples.

Runtime and browser suites were not run because this PR only changes Markdown guidance.

[github-actions]

prepare-merge: no package or runtime-affecting changes; package staging and durable package publishing were skipped. merge-gate=success posted on PR HEAD. This PR is ready for a reviewer to squash merge.