docs: document release and RC workflow

[okwn]

Summary

Documents the release and RC workflow in CONTRIBUTING.md:

• Release schedule for dev (nightly), preview (weekly), stable (biweekly) channels
• How to cut release candidates via the Cut New Release Candidate workflow
• How to test RCs (beta download links, Slack coordination)

Why this matters

Contributors don't know when or how releases happen. Documenting the channel model and RC process makes it clear who does what and when.

Changes

• Add Release and rc workflow section to CONTRIBUTING.md
• Document release channels, schedule, and RC cut process
• Add guidance for testing and signing off on RCs

Validation

Read the CONTRIBUTING.md release section and verify it matches actual workflow.

Risk/Compatibility

Documentation only.

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Closes #11384 (Make CLI agent examples clickable in README)

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@okwn

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[oz-for-oss]

@okwn

This PR is not linked to an issue that is marked with ready-to-implement.

Issue-state enforcement details:

• Associated same-repo issues checked: none

• Required readiness label: ready-to-implement

To continue, link this PR to a same-repo issue such as Closes #123 in the PR description, and make sure that issue has ready-to-implement.

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