[codex] Add schedule preparation flow SRD

[jjoonleo]

Summary

• Add a Schedule Preparation Flow SRD covering early start, official start, delayed entry, runtime ticking, finish, persistence, and cleanup behavior.
• Link the SRD from the documentation index.
• Document follow-up mismatches as GitHub issues Align schedule start prompt variants with early-start SRD #361, Handle exact preparation start boundary in ScheduleBloc #362, Return alarm screen home when no active schedule exists #363, and Show ready state after completing preparation before leave time #364.

Validation

• Docs-only change; Flutter tests were not run.

[github-actions]

Visit the preview URL for this PR (updated for commit 8ffc334):

https://ontime-c63f1--pr365-codex-schedule-prepa-glptkoz2.web.app

_{(expires Tue, 12 May 2026 05:40:28 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

_{Sign: 7d60a2eae4c4a5839220ed6a57552a3b61cce5a3}

[github-actions]

Visit the preview URL for this PR (updated for commit 8ffc334):

https://on-time-front-widgetbook--pr365-codex-schedule-prepa-nv3hxk03.web.app

_{(expires Tue, 12 May 2026 05:40:30 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

_{Sign: e53128a376fed0209d449279de9c0d94c83cdd8b}