feat(fleet): add durable tmux fleet poc

packages/mosaic/framework/systemd/user/README.md

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+# Mosaic tmux Fleet PoC
+
+This directory contains the first durable tmux-backed fleet primitives for the
+Mosaic software-factory model.
+
+The lifecycle model follows the organization-neutral AI Guide playbook
+`mosaicstack/aiguide:playbooks/tmux-fleet.md` (commit `2a0b0b5`): a dedicated
+holder owns the tmux server/socket; agent units join it and stop only their own
+exact-match session.
+
+## Layout
+
+- `mosaic-tmux-holder.service` — user-mode holder that owns the named tmux server.
+- `mosaic-agent@.service` — user-mode template for one reusable agent session.
+- `test-fleet-units.sh` — validates unit syntax and required relationships.
+
+The agent template calls:
+
+```text
+~/.config/mosaic/tools/fleet/start-agent-session.sh <agent-name>
+```
+
+which starts or reuses a tmux session on `MOSAIC_TMUX_SOCKET`.
+
+## Local customization
+
+Per-agent overrides live outside the package in:
+
+```text
+~/.config/mosaic/fleet/agents/<agent>.env
+```
+
+Example:
+
+```dotenv
+MOSAIC_TMUX_SOCKET=mosaic-factory
+MOSAIC_AGENT_RUNTIME=claude
+MOSAIC_AGENT_WORKDIR=/home/jarvis/src/mosaic-stack
+# Optional escape hatch for PoC/canary agents:
+# MOSAIC_AGENT_COMMAND=mosaic yolo claude
+```
+
+## Manual canary sequence
+
+```bash
+mkdir -p ~/.config/systemd/user ~/.config/mosaic/tools/fleet ~/.config/mosaic/fleet/agents
+cp packages/mosaic/framework/systemd/user/mosaic-*.service ~/.config/systemd/user/
+cp packages/mosaic/framework/tools/fleet/start-agent-session.sh ~/.config/mosaic/tools/fleet/
+chmod +x ~/.config/mosaic/tools/fleet/start-agent-session.sh
+systemctl --user daemon-reload
+systemctl --user start mosaic-tmux-holder.service
+systemctl --user start mosaic-agent@canary.service
+tmux -L mosaic-factory ls
+```
+
+Do not use `tmux kill-server` without `-L mosaic-factory`; this pattern is meant
+to avoid disturbing the user's default tmux server.