docs: stop pointing contributors at the repositories being archived

[bgm-malbeclabs]

Step 6 of the monorepo migration, the half that lives in this repository. Archiving the two source repositories is an action on those repositories, and the text to use is below. Stacked on #4257, so the diff shown here is this change alone.

Summary

• offchain/CONTRIBUTING.md is removed. It told contributors to fork doublezero-offchain and to open pull requests and issues against it. Both stop being true the moment that repository is archived: an archived repository takes no issues and no pull requests. Everything else in the file repeated the root README, and the one thing the root lacked, the Contributor Covenant reference, moves there.
• DEVELOPMENT gains a section on the two imported trees, naming what the root make targets do not reach: the Solana L1 programs, which keep their own workspace, lockfile and 1.91 toolchain so their bytes stay reproducible against sha256sums_*.txt, and the Elixir scheduler. Both now have their own path-scoped CI, which the section names.

I checked the rest of both trees for pointers that stop being true after the archive. There is only one other class, and it should stay: the per-crate changelogs link to releases at doublezero-offchain. Those releases exist there and an archived repository still serves them, so the links keep working. Rewriting historical release links would make them wrong, not right.

For the archive itself

Both repositories keep their transfer redirects, their 11 forks and their old release download URLs, so archive them read-only rather than deleting. Suggested notice for the top of each README before archiving:

This repository has moved. The code now lives in
malbeclabs/doublezero, under offchain/,
with its full history and its release tags. This repository is archived and read-only.
Open issues and pull requests against malbeclabs/doublezero.

Existing releases and their download URLs continue to resolve here. New releases are
published from malbeclabs/doublezero.

Same text for doublezero-solana, with solana/ as the directory.

Archiving is also what unblocks step 7, and the two are deliberately not the same day: an archived repository still serves reads, so doublezero-shreds keeps resolving malbeclabs/doublezero-solana at tag revenue-distribution/v0.3.7, and that tag imported here points at the same commit. Shreds can be repointed when it suits.

Testing Verification

• Searched both imported trees for links to the source repositories outside changelogs and release links. offchain/CONTRIBUTING.md was the only live pointer; offchain/Cargo.toml also carried repository and homepage fields naming the old repo, and workspace: fold the offchain crates into the root workspace #4255 deletes that file outright.

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[Copilot]

🟡 Changes recommended

The new DEVELOPMENT instructions do not match the actual Elixir CI steps, and the README Code of Conduct link should use HTTPS.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates contributor-facing documentation for the monorepo migration by removing guidance that will become incorrect once the source repositories are archived, and by documenting how to work with the imported offchain/ and solana/ trees inside this repository.

Changes:

• Move the Contributor Covenant reference from offchain/CONTRIBUTING.md into the root README.md, and delete the obsolete contributing file.
• Add a new “imported trees” section to DEVELOPMENT.md that explains what root make targets do not cover and which CI workflows apply.
• Record the documentation changes in CHANGELOG.md.

File summaries

File	Description
README.md	Adds a project-wide Code of Conduct reference for contributors.
offchain/CONTRIBUTING.md	Removes outdated contribution instructions that point to soon-to-be-archived repos.
DEVELOPMENT.md	Documents how to run checks for the imported solana/ and Elixir scheduler trees.
CHANGELOG.md	Notes the documentation updates related to the archive and imported-tree workflows.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 2
• Review effort level: Lite

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