docs: update production sync testing to require 60 seconds

The sync system uses a 5-second batch window for discovered repos.
Repos discovered late in a 30-second test don't have enough time for
the full Layer 2→3→4 cascade:

- Layer 1: Discover repo announcements (0-5s)
- Layer 2: Send #a, #A, #q filters for repos (5-30s)
- Layer 3: Receive issues, patches, PRs (30-60s)
- Layer 4: Receive comments on root events (40-60s)

Testing confirmed that 60 seconds allows late-discovered repos
(gitworkshop, ngit) to complete all layers, while 30 seconds only
allows 1 second after Layer 2 filters are sent.

Updated all references from 30s to 60s throughout the guide and added
explanation of why this duration is necessary.

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