Document abbreviate_rule's nat/rdr blind spot and ddclient's log gapHEADmaster

From a fresh-agent review of AGENTS.md against two hypothetical tasks:

- abbreviate_rule() in pf.sh has only ever been developed/tested
  against filter-rule (-sr) syntax, despite also running on -sn
  (nat/rdr) output. It degrades safely (unshortened passthrough) but
  a new nat/rdr rule shape isn't guaranteed to compress as tightly -
  noted both in the code comment and in AGENTS.md's compression
  writeup so this doesn't need rediscovering.

- ddclient logging isn't enabled on the box, so "is the last DNS
  update actually succeeding" can't be built by tailing a log that
  doesn't exist - it'd need the cache file or a public-IP comparison
  instead. Recorded in AGENTS.md's topology section since ddclient.sh
  currently only checks the process is running, not update success.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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