v0.42.3 — repo cleanup

Repository cleanup: unused code removed and documentation reconciled with the
source. No add-on behaviour changes.

• Removed three unused helpers with no callers — ami.get_contacts,
  ami.get_channels, and mwi_store.all_flags — each superseded by
  ami.get_status_bundle / mwi_store.exts. Their underlying *_from_blocks
  parsers remain in use and are unaffected.
• Dropped an obsolete DOCS.md §14 troubleshooting row describing a web-terminal
  ValueError at start as harmless; that startup race was eliminated in 0.29.1, so
  such a ValueError now indicates a genuine fault rather than a benign one.
• Removed the inert armv7 base image from build.yaml; the manifest, README, and
  DOCS all declare amd64/aarch64 only.
• Corrected a stale switchboard-config comment that described the tmpfs AMI secret
  file as 0600; it is created 0640, as SECURITY.md already documents.
• Genericized the SIP-trunk POP hostname in the test fixtures to losangeles.voip.ms,
  matching the documentation and completing the 0.42.2 hygiene pass.