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.

packages/mosaic/framework/systemd/user/mosaic-agent@.service

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+[Unit]
+Description=Mosaic tmux fleet agent %i
+Documentation=https://git.mosaicstack.dev/mosaicstack/stack
+Requires=mosaic-tmux-holder.service
+After=mosaic-tmux-holder.service
+PartOf=mosaic-tmux-holder.service
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+Environment=MOSAIC_TMUX_SOCKET=mosaic-factory
+Environment=MOSAIC_AGENT_NAME=%i
+Environment=MOSAIC_AGENT_RUNTIME=pi
+Environment=MOSAIC_AGENT_WORKDIR=%h
+EnvironmentFile=-%h/.config/mosaic/fleet/agents/%i.env
+ExecStart=/bin/bash %h/.config/mosaic/tools/fleet/start-agent-session.sh %i
+ExecStop=-/bin/bash -lc 'tmux -L "${MOSAIC_TMUX_SOCKET:-mosaic-factory}" kill-session -t "=%i"'
+
+[Install]
+WantedBy=default.target

packages/mosaic/framework/systemd/user/mosaic-tmux-holder.service

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+[Unit]
+Description=Mosaic tmux fleet holder
+Documentation=https://git.mosaicstack.dev/mosaicstack/stack
+After=default.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+Environment=MOSAIC_TMUX_SOCKET=mosaic-factory
+Environment=MOSAIC_TMUX_HOLDER=_holder
+ExecStart=/bin/bash -lc 'tmux -L "$MOSAIC_TMUX_SOCKET" has-session -t "=${MOSAIC_TMUX_HOLDER}:0.0" 2>/dev/null || tmux -L "$MOSAIC_TMUX_SOCKET" new-session -d -s "$MOSAIC_TMUX_HOLDER" "while true; do sleep 3600; done"'
+ExecStop=-/bin/bash -lc 'tmux -L "$MOSAIC_TMUX_SOCKET" kill-server'
+
+[Install]
+WantedBy=default.target

packages/mosaic/framework/systemd/user/test-fleet-units.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR=$(cd -- "$(dirname -- "$0")" && pwd)
+HOLDER="$SCRIPT_DIR/mosaic-tmux-holder.service"
+AGENT="$SCRIPT_DIR/mosaic-agent@.service"
+
+fail() {
+  echo "FAIL: $*" >&2
+  exit 1
+}
+
+[ -f "$HOLDER" ] || fail "missing mosaic-tmux-holder.service"
+[ -f "$AGENT" ] || fail "missing mosaic-agent@.service"
+
+grep -qF 'ExecStart=' "$HOLDER" || fail "holder has no ExecStart"
+grep -qF 'tmux -L' "$HOLDER" || fail "holder does not use named tmux socket"
+grep -qF '_holder' "$HOLDER" || fail "holder session is not explicit"
+grep -qF 'Requires=mosaic-tmux-holder.service' "$AGENT" || fail "agent does not require holder"
+grep -qF 'start-agent-session.sh' "$AGENT" || fail "agent unit does not call start-agent-session.sh"
+grep -qF 'kill-session -t "=%i"' "$AGENT" || fail "agent stop does not exact-match its session"
+
+if command -v systemd-analyze >/dev/null 2>&1; then
+  systemd-analyze verify --user "$HOLDER" "$AGENT" >/tmp/mosaic-fleet-systemd-verify.log 2>&1 || {
+    cat /tmp/mosaic-fleet-systemd-verify.log >&2
+    fail "systemd-analyze verify failed"
+  }
+fi
+
+echo "ok - fleet systemd unit templates"