Clarify gate-off docs: an existing assignee is still read and shown

Copilot review on PR #15: the 'gate off' wording in AGENTS.md and
.env.example read as 'no assignee', but read_task() always parses the
Assignee line and the card face always renders it. The gate governs only
whether a move writes/clears the line and commits. Reword both to say so.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

manager/core/.env.example

@@ -63,8 +63,11 @@ BOARD_FETCH_TIMEOUT=10
 # backlog/ clears it, and each move commits itself — the move and the claim
 # in one commit touching only that task file, messaged
 # `board: <number> → <stage> (<name>)`. Nothing is pushed. Off (the
-# default) means today's behaviour exactly: no assignee, no commit, tasks/
-# committed by hand. Anything but empty/0/false/no/off turns it on.
+# default) means moves neither write nor clear the assignee and never
+# commit — today's behaviour exactly, tasks/ committed by hand. An
+# assignee added to a file by hand is still read and shown either way;
+# the gate only governs whether a move writes it. Anything but
+# empty/0/false/no/off turns it on.
 BOARD_COMMIT_MOVES=
 
 # Seconds between disk polls of the stage directories.