refactor(common): delete the sender-key implementation nothing uses

`senderkeys.py` and its 13 tests implemented Signal-style sender keys, and
production has never called them: chat is a key per group, per epoch, per
device, derived by name. The reasoning that ruled the ratchet out stays where it
belongs — in `chatbox.py`, at the top of the module that replaced it — because
the argument is the useful part, and it now stands on its own instead of
pointing at a file to compare against.

Kept code that nothing calls is worse than absent code: it reads as an
alternative somebody may reach for, and it has to be maintained past every
refactor to stay compiling, which is maintenance spent on a decision already
made.

The three comments naming `GroupSenderKeyStore` are rewritten to say the thing
they were illustrating.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

1 files changed, 3 insertions, 5 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/chat/__init__.py b/packages/meshbay-node/src/meshbay_node/chat/__init__.py
index cb2c868..400cb1a 100644
--- a/packages/meshbay-node/src/meshbay_node/chat/__init__.py
+++ b/packages/meshbay-node/src/meshbay_node/chat/__init__.py
@@ -1,10 +1,8 @@
 """MeshBay Node — chat storage and relay.
 
-Encryption is the client's: the node holds an epoch key it delivers to members
-and never a plaintext message once a group has the switch on. See
-`docs/chat-sender-keys.md`. It is not the Sender Keys ratchet this module's
-docstring used to name — `senderkeys.py` is unused by production and is kept for
-a possible future 1:1 DM, alongside `ratchet.py`.
+Encryption is the client's: the node holds an epoch key it delivers to members,
+and never a plaintext message. Not a ratchet — a key per group, per epoch, per
+device, for the reasons `meshbay_common/chatbox.py` sets out.
 """
 from .store import (
     FORMAT_PLAIN,