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, 4 insertions, 6 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
index ec7ef5d..2a90bb4 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
@@ -383,12 +383,10 @@ class WebRTCPeerSession:
         self._username: str = ""
         # This connection's key in the group's peer registry. **Per connection,
         # never per account**: one person may hold several devices here, and
-        # keying the registry by user_id made the second evict the first — the
-        # same "keyed by account where it should be keyed by device" mistake as
-        # `pin_identity`'s old INSERT OR REPLACE and as GroupSenderKeyStore's
-        # silent overwrite. Symptom was invisible: two devices of one account
-        # could not both be connected, and whichever disconnected took the
-        # other's chat delivery with it. See docs/chat-sender-keys.md F7.
+        # keying the registry by user_id makes the second evict the first, and
+        # the symptom is invisible: two devices of one account cannot both be
+        # connected, and whichever disconnects takes the other's chat delivery
+        # with it.
         self._registry_key: str = uuid.uuid4().hex
         # Set from the roster: the key this node pinned for this account. Never
         # from the JWT — the hub picks what goes in there.