feat(fleet): add local canary CLI (#563)

docs/guides/fleet-local-canary.md

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+# Local Fleet Canary
+
+The local fleet canary runs a small tmux-backed Mosaic agent fleet on an
+isolated tmux socket. The default socket is `mosaic-factory`; the commands do
+not use or stop the default tmux server.
+
+## Files
+
+Product-owned defaults:
+
+- `packages/mosaic/framework/fleet/roster.schema.json`
+- `packages/mosaic/framework/fleet/examples/minimal.yaml`
+- `packages/mosaic/framework/fleet/examples/local-canary.yaml`
+- `packages/mosaic/framework/systemd/user/mosaic-tmux-holder.service`
+- `packages/mosaic/framework/systemd/user/mosaic-agent@.service`
+- `packages/mosaic/framework/tools/fleet/start-agent-session.sh`
+- `packages/mosaic/framework/tools/tmux/agent-send.sh`
+- `packages/mosaic/framework/tools/tmux/send-message.sh`
+
+Site-owned local roster:
+
+```text
+~/.config/mosaic/fleet/roster.yaml
+```
+
+Do not put a host-specific full roster into product defaults. Start from an
+example and edit the local roster after `mosaic fleet init --write`.
+
+## Install
+
+Minimal canary:
+
+```bash
+mosaic fleet init --profile minimal --write
+# If a site-owned roster already exists, inspect it first; overwrite only explicitly:
+# mosaic fleet init --profile minimal --write --force
+mosaic fleet install-systemd
+systemctl --user daemon-reload
+mosaic fleet start
+mosaic fleet verify
+```
+
+Small dogfood roster:
+
+```bash
+mosaic fleet init --profile local-canary --write
+# Use --force only after preserving any site-owned roster changes.
+mosaic fleet install-systemd
+systemctl --user daemon-reload
+mosaic fleet start
+mosaic fleet status
+```
+
+## Agent Operations
+
+```bash
+mosaic agent roster
+mosaic agent status
+mosaic agent status canary-pi
+mosaic agent send canary-pi --message "status check"
+mosaic agent reset canary-pi --new
+mosaic agent tail canary-pi -n 80
+```
+
+These commands read the roster and target the configured tmux socket. The
+generated systemd agent services use `start-agent-session.sh`; message delivery
+uses the tmux send tools with `-L mosaic-factory`.
+
+## Verification
+
+Use these checks before expanding the roster:
+
+```bash
+tmux -L mosaic-factory ls
+tmux ls
+mosaic fleet verify
+systemctl --user status mosaic-tmux-holder.service
+```
+
+Expected results:
+
+- `tmux -L mosaic-factory ls` shows `_holder` and roster agent sessions.
+- `tmux ls` shows only the default tmux server sessions and is not changed by
+  fleet start/stop operations.
+- `mosaic fleet verify` checks exact session targets on the isolated socket.
+
+## Rollback
+
+Stop the local canary:
+
+```bash
+mosaic fleet stop
+systemctl --user disable mosaic-agent@canary-pi.service
+systemctl --user disable mosaic-tmux-holder.service
+systemctl --user daemon-reload
+```
+
+For a full local cleanup of generated canary files:
+
+```bash
+rm -f ~/.config/systemd/user/mosaic-agent@.service
+rm -f ~/.config/systemd/user/mosaic-tmux-holder.service
+rm -rf ~/.config/mosaic/fleet
+rm -rf ~/.config/mosaic/tools/fleet
+```
+
+This rollback leaves the default tmux server untouched. If a canary session is
+still present after service stop, remove only the isolated socket server:
+
+```bash
+tmux -L mosaic-factory kill-server
+```