Relay compare-and-set for sidebar sections snapshot writes #8805

Problem

buzz sidebar sections (added in #8759) edits the channel sections preference (kind:30078, d=channel-sections) with a one-shot read-then-publish. Each command fetches the current head, applies its edit, and publishes the full snapshot as a NIP-33 replacement.

Two writers that read the same head before either publishes both produce a complete document derived from the old head. The relay accepts both as replacements. The one with the later created_at (or the lower id within the same second) wins, and the other writer's changes disappear. Both commands report success.

Review thread: #8759 (comment)

Who is affected

• One-shot CLI writers: two CLI runs, or a CLI run and the desktop app, can silently overwrite each other.
• Desktop mitigates this by keeping a pending edit and merging live head events. A one-shot CLI process cannot do that.

#8759 documents the limitation in the sidebar sections help text, the CLI README, and the nest skill.

Proposed fix

Add a relay-side compare-and-set check for sections snapshot writes:

• The writer sends the id of the head event it read (the expected current head).
• The relay rejects the publish when the stored head has a different id.
• The CLI maps the rejection to exit code 5 (write conflict) so the caller can re-read and retry.
• Desktop can send the same expected head and keep its pending-merge flow on conflict.

This also needs a decision on how the expected head travels with the event (for example a tag) and how it behaves for the first write, when no head exists.

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