feat(fleet): add durable tmux fleet poc

packages/mosaic/framework/tools/tmux/README.md

@@ -31,9 +31,12 @@ Prepends the preamble automatically (auto-detecting your own `host:session`) and
 delivers reliably to local OR remote panes.
 
 ```bash
-# Local target (same host)
+# Local target (same host, default tmux server)
 agent-send.sh -s <dst_session> -m "message"
 
+# Local target on a Mosaic fleet socket
+agent-send.sh -L mosaic-factory -s '=coder0' -m "message"
+
 # Remote target (over ssh)
 agent-send.sh -H user@host -s <dst_session> -m "message"
 
@@ -42,10 +45,27 @@ agent-send.sh -H user@host -s <dst_session> -f msg.txt
 echo "msg" | agent-send.sh -s <dst_session>
 ```
 
-Key flags: `-s` dst session (required) · `-H` ssh target for remote · `-n` dst
+Key flags: `-L` named tmux socket · `-s` dst session (required) · `-H` ssh target for remote · `-n` dst
 hostname for the preamble (else auto-resolved) · `-m`/`-f`/stdin body · `-S`
 override source label · `-v` verbose · `-r N` Enter-flush attempts.
 
+For durable fleet use, prefer exact tmux targets such as `=coder0`. The helper
+normalizes exact session targets to pane-qualified targets internally so pane
+commands do not fall back to tmux's prefix matching behavior.
+
+## Named socket isolation
+
+Durable Mosaic fleets should use a dedicated tmux socket, for example:
+
+```bash
+tmux -L mosaic-factory ls
+agent-send.sh -L mosaic-factory -s '=coder0' -m "status?"
+send-message.sh -L mosaic-factory -t '=coder0' -m "raw pane message"
+```
+
+This keeps fleet operations away from the user's default tmux server. It is the
+safe rollout path on hosts that already have manual tmux sessions.
+
 ## Why a helper exists (the submission gotcha)
 
 Pasting into an interactive REPL via raw `tmux send-keys` is unreliable: a
@@ -67,6 +87,7 @@ message crosses the wire as base64 (`-b`) to avoid all shell-quoting hazards.
 
 - `agent-send.sh` — inter-agent wrapper (preamble + local/remote dispatch).
 - `send-message.sh` — low-level reliable single-pane submitter (`-b` base64 input).
+- `test-send-message-socket.sh` — smoke test for named-socket isolation.
 
 ## Distribution