Review M2b: accept with one follow-up task; record lessons

All ten tasks pass the checklist, the gate, the audit and the device
checks, including a four-turn conversation with a loopd restart and no
cache loss. Reading and probing found four low defects: the busy guard
is released before the final frame on the main path but not on the
three error paths, its Drop skips a poisoned lock, an unreadable
core.md is treated as missing, and bxctl's interactive loop exits on a
failed turn. Task 11 carries the fixes with two new tests, checked
against a fixed copy of the branch.

The Model column is filled in (all Ornith) and one malformed row is
repaired. Two rules are promoted to AGENTS.md.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

AGENTS.md

@@ -61,6 +61,9 @@ These come from defects found in review. The evidence is in `docs/implementer-le
 - If a tool you were told to use does not exist, stop and say so. Do not invent a command in its
   place.
 - A `read` returns as soon as it has any data to give and blocks only when it has none.
+- A rule about one path applies to every path that does the same thing, including early returns
+  and error paths the task did not walk through.
+- A file that exists but cannot be read is an error. Only a missing file may count as absent.
 
 ## The gate