Friday demo: board-header auto-merge toggle + demo script

[deco-cms]

The task is the Friday demo run-through: connect integrations → generate backlog → agent works cards → In Review → open card / see PR → Edit opens CMS → Publish → toggle auto-merge at the top → card goes to Done.

Walking the flow against the product, every beat already exists except the last one: auto-merge is an org flag that only lives in Settings → Review, not "at the top" of the board. So this PR does two things:

1. apps/web/src/layouts/task-board/index.tsx — adds an Auto-merge switch to the task board header toolbar (right side, next to the view toggle). It reads/writes the same auto_merge org flag as Settings → Review via useOrgFlag/useSetOrgFlag, so flipping it from either place is the same setting. Errors toast. EN + pt-BR strings added.
2. docs/friday-demo-script.md — the demo script itself: pre-flight checklist (notably: auto-merge must start OFF, since turning it on is the closing beat), seven timed beats with the lines to say, and a fallback table for the ways each beat can break on stage.

No behaviour change to merging itself — the toggle is a second surface for an existing flag.

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Summary by cubic

Adds an Auto-merge switch to the task board header so the org’s auto_merge flag can be toggled in-context during reviews. Previously this flag was only in Settings → Review; now it’s also in the board header. No change to merge behavior.

• Reads/writes the same auto_merge flag via useOrgFlag/useSetOrgFlag; shows a toast on update error.
• Renders next to the view toggle in apps/web/src/layouts/task-board/index.tsx using @decocms/ui Switch and GitMerge icon; includes title, label, and aria-label.
• Adds EN and pt-BR i18n keys for the label, tooltip, and error.

^{Written for commit cdc61d4. Summary will update on new commits.}