feat(nodes): SSH bootstrap + housekeeping — install script, bundled binaries, auto-prune

- `containr nodes add <user@host>` mints an enroll token, SSHes once,
  runs the install script remotely; --print emits the one-liner
- Public /api/agents/install.sh + /api/agents/download/{platform};
  agent binaries built into the image under /agents
- Agent-side `prune` (bounded docker system prune, volumes opt-in)
  and `system_df` commands
- node_agents.auto_prune + health sweep enqueues daily prune;
  offline/online transitions notify admins
- CLI nodes update/prune/commands + MCP tools (prune confirm-gated)
- Usage page: install one-liner, per-node Prune + auto toggle

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/migrations_goose/20261107000000_agent_housekeeping.sql

@@ -0,0 +1,5 @@
+-- +goose Up
+ALTER TABLE node_agents ADD COLUMN IF NOT EXISTS auto_prune BOOLEAN NOT NULL DEFAULT false;
+
+-- +goose Down
+ALTER TABLE node_agents DROP COLUMN IF EXISTS auto_prune;