Polish OSS contribution standards in CONTRIBUTING.md

[theyostalservice]

Why

Our CONTRIBUTING.md lacked clear standards around contribution quality, which has led to increased review burden — PRs mixing concerns, missing tests, missing changelog entries, and interface changes arriving without prior discussion.

Closes DI-3578.

What

• New section: "Before you write any code" — requires contributors to open a GitHub issue and discuss with the Semantic Layer engineering team before implementing any interface change (APIs, schemas, CLI commands, etc.). PRs without prior discussion on interface changes will be declined.
• Lint is now a hard pre-PR requirement — reframed from "run at any time" to a blocking requirement, with CI enforcement called out explicitly.
• Tests are required — added explicit callout that PRs adding behavior or fixing bugs must include tests; reviewers will ask for them if missing.
• PR scope guidance strengthened — added bold callout that a PR should do one thing, with concrete heuristic ("also" / "while I was in here" = signal to split); added guidance to use commits to tell the story of a PR.
• Rebase step clarified — replaced vague "merge your changes into your fork" with explicit git fetch upstream && git rebase upstream/main instruction.
• Changelog timing fixed — corrected "once your PR is created" to "before opening your PR".

Notes

Paul Yang should do a final review and improvement pass per the Jira ticket.

Drafted by Claude Sonnet 4.6 under the direction of @theyostalservice

[linear]

DI-3578 Tab name for query

[plypaul]

🚢