Clarify draft PR lifecycle in maintainer workflow

[karthiknadig]

Problem

The maintainer workflow says to create a draft PR, but it does not explicitly say when to transition it to ready. Agents can mark a PR ready immediately after creation, before Copilot review, CI, and quality snapshot inspection are complete.

Acceptance criteria

• Keep maintainer-created PRs in draft throughout implementation, review feedback, CI, and quality snapshot inspection.
• Mark the PR ready only after no actionable review comments remain and required checks/snapshots are acceptable.
• Enable merge or auto-merge only after the ready transition and merge prerequisites are satisfied.