docs(working): add a human-facing index + plan status reference

CLAUDE.md keeps the status convention as a one-line rule (it has to be
loaded to be followed). This is the same convention written out for a
human reader, with the meanings the trim dropped, plus what each
subfolder of docs/working/ is for and a grep one-liner for "what's open".

Notes the two distinctions that matter in practice: Deferred is not
Abandoned, and a trailing note after "Complete" is normal.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/README.md

@@ -8,7 +8,7 @@
 - [Switching to a new trip](guides/trip-switching.md)
 - [Rebuilding local dev from scratch](guides/local-setup.md)
 
-**Checking project status?** → [`working/`](working/)
+**Checking project status?** → [`working/`](working/) — [what's in there + the plan status convention](working/README.md)
 - [Backlog](working/backlog.md)
 - [Bugs and fixes](working/bugs-and-fixes.md)
 - [QA results](working/qa/results.md)