fix(desktop): queue agent draft requests

[kiranmagic7]

Summary

Fixes #3559.

The desktop previously marked every accepted agent management request as seen before checking whether another draft was open. Requests arriving during an active review were therefore discarded permanently.

This change keeps accepted requests in a FIFO queue and advances to the next draft after the current one is saved or dismissed. The requesting agent pubkey follows each queued item, and request errors are cleared between drafts.

Tests

1. pnpm --dir desktop exec node --experimental-strip-types --test src/features/agents/agentManagementBuffer.test.mjs
2. pnpm --dir desktop typecheck
3. just ci

Notes for review

The queue is intentionally not capped. These frames are already authenticated and owner scoped, and introducing a cap would preserve the silent data loss described in the issue.

[kiranmagic7]

Status check on head 5b8d967411a7c55b8a4a97eaa555a079e498a571: the PR is still mergeable, with DCO Check, Semgrep OSS, and zizmor green. The full Actions run is waiting on fork workflow approval rather than failing code: https://github.com/block/buzz/actions/runs/30485508497 (action_required, 0 jobs).

Review focus: the small draft-request buffer in agentManagementBuffer.ts and its regression coverage in agentManagementBuffer.test.mjs for #3559. The repo CODEOWNERS path is already @block/buzz-oss-team, so I am leaving routing as-is.