fix(coord): mark broadcast messages as read on server

PROBLEM: Broadcast messages were never being marked as read on the server,
only tracked in a local gitignored seen-file. This caused them to re-appear
in every new session or on different machines.

ROOT CAUSE: check-messages.sh lines 101-104 had a flawed assumption that
broadcasts share a single read_at field that would "clobber" other machines'
unread state. This was wrong - the API supports per-session read tracking.

FIX:
- check-messages.sh now marks broadcasts as read on the server (like personal
  messages), in addition to tracking them in the local seen-file
- Updated comments to reflect correct behavior
- coord SKILL.md now documents auto-mark-read behavior and reply workflow
- Manually marked all 39 accumulated unread broadcasts as read

IMPACT: Broadcast messages will now be properly marked as read and won't
keep appearing across sessions. Fixes user complaint about answered questions
(pfSense cred-path, fabb3421, etc.) continuing to show up.

Logged to errorlog.md as --correction.

errorlog.md

@@ -17,6 +17,8 @@ Categories (the `[type]` tag): _(none)_ = skill/command execution failure ·
 
 <!-- Append entries below this line -->
 
+2026-06-22 | Mikes-MacBook-Air.local | coord/check-messages.sh | [correction] broadcasts never marked read on server, only in local seen-file -> repeat on every session [ctx: fix: mark broadcasts read on server like personal messages]
+
 2026-06-21 | Howard-Home | git/submodule | [friction] Did feature work directly in the SHARED guru-rmm submodule working tree while a CONCURRENT Claude session was active in it. The other session switched branches (fix/audit-cleanup -> bugfix/bug-019 -> detached) and repointed my branch ref mid-work, and the working tree ended up with BOTH sessions' uncommitted changes mixed together. Wasted a recovery cycle. FIX: when doing submodule feature work and other sessions may be live (the /save note warns 3-4 sessions share one tree), create an isolated 'git worktree add <path> origin/main' FIRST, do all edits + commit + push-by-SHA there, then 'worktree remove' — never rely on the shared checkout's branch/HEAD surviving. Do NOT 'git checkout --' shared files to clean up (clobbers the other session's uncommitted work). [ctx: ref=git/submodule detached-HEAD + stale-audit friction]
 
 2026-06-21 | Howard-Home | bitdefender | GravityZone API error [policies.getPolicyDetails]: Invalid value for 'policyId' parameter. [ctx: cmd=policy]