[Docs] Sync recent user-facing behavior

[roomote-community]

​Created by Roomote. View the task or mention @roomote for follow-up asks.

What changed

• Update Session launch, model, review, artifact, and Live Preview guidance to match current behavior.
• Document reserved deployment-variable safeguards and source-control Fast reply presentation.
• Clarify Merge Announcer pull request labels and links.

Why this change was made

Several user-facing changes merged during the previous 24 hours were missing from the public docs, and older dashboard and Build this instructions no longer matched the shipped UI.

Impact

Users now get current guidance for starting Sessions, choosing an environment, acting on artifacts, following previews and reviews, configuring environment variables, and understanding source-control responses. The docs validation and broken-link checks pass; refreshed browser proof could not complete within the capture deadline after the final consistency edit.

[roomote-community]

1 issue outstanding. See task

• Correct the nonexistent post-create/edit Use environment flow in apps/docs/fast-sessions.mdx:32 and apps/docs/how-roomote-works.mdx:87.

_{Reviewed f54252e}

[roomote-community]

There is currently no Use environment action after creating or editing an environment. Creation redirects directly to /?environmentId=..., while editing returns to the environments list; onGoUseIt is not rendered by EditEnvironmentPage. This makes the new instructions here and in how-roomote-works.mdx impossible to follow. Document the actual redirect/launcher flow, or add the described action.