fix(board): track an open HTML comment through to its closer

Skipping lines that START with the comment opener is not the same as
skipping a comment. Every continuation line of a wrapped comment under
the NESTE heading still looked like prose, so the excerpt became comment
internals instead of the next step. Both the board line and the route
line wrap easily; measured, 1 of 28 repos was affected, with the symptom
held down only by the convention of keeping those comments one-line.

An open comment is now tracked to its closer, and a NESTE block that is
nothing but a comment reports an empty block rather than promoting the
comment's own text.

Two limits are the format, not leftovers: an HTML comment body may not
contain the closer at all, so a rationale quoting it still ends its own
comment early.

The fixture is the real shape - one-line, wrapped, one-line, prose -
because a lone wrapped comment passes even with a flag that never
resets, and the continuation marker sits at the start of its line
because the table's 38-char cut would otherwise truncate it away and
the check would pass against the broken code too.

board-selftest 138 -> 142.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014K262DRWBJzEpWoMsBjB8J

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "repo-mailbox",
-  "version": "0.20.0",
+  "version": "0.20.1",
   "description": "Local mailbox for coordination between Claude Code sessions in different repositories. Directed messages and broadcasts as plain Markdown files on your own disk, injected as context at session start. Local, private, no network.",
   "author": {
     "name": "Kjell Tore Guttormsen"