(CODUX-125) Synchronize runbook with conditional update button behavior - #2772

## Objective
Bring the canonical operational documentation in sync with the shipped Electron title-bar update behavior.

## Scope
- Edit `docs/operations/runbook.md`.
- If the same title-bar guidance is mirrored in any public docs page under `docs-web/`, update that copy too.
- Keep unrelated runbook content unchanged.

## Requirements
1. Replace the stale note that says the Electron title bar always shows an Update button whenever `window.codeUxDesktop.window` is available.
2. Document that the Update affordance only appears when `useUpdateStatus()` reports `updateAvailable === true`.
3. Note that the button remains version-aware when a latest version is known, and the visible action still calls the existing `openUpdates()` bridge.
4. Remove any wording that implies the title bar ignores `/api/system/update-status`.
5. Keep the explanation of the update-status pipeline accurate and concise for operators.

## Verification
- Run a repository search and confirm there are no remaining stale references such as `always-present "Update" button` or `does not depend on /api/system/update-status` in the docs tree.
- Manually confirm the runbook now matches the shipped title-bar behavior.