feat(nodes): agent self-upgrade (#57)

POST /agents/:id/upgrade enqueues a self_upgrade command carrying the
target platform, the sha256 the server computed over its bundled binary,
and the server version. The agent downloads the replacement from the
existing /api/agents/download/:platform endpoint, verifies the checksum,
sanity-checks the new binary (--version), atomically swaps its own
executable, reports the command result, then re-execs via syscall.Exec —
no supervisor or restart plumbing required.

Agents stamp main.version via ldflags at image build, expose --version,
and now send version at register so node_agents.version refreshes on
re-register (older agents without the field keep their stored value).
The server rejects upgrades for offline nodes and for platforms with no
bundled binary (linux-amd64/linux-arm64 ship in the image).

Surfaces: nodes upgrade CLI, containr_nodes_upgrade MCP (confirm-gated),
node-detail page button, OpenAPI. Verified live: v0.0.3 agent upgraded
to v0.0.4 in place, re-registered with the new version; offline node
returns 409 NODE_OFFLINE.

Generated with [Devin](https://devin.ai)

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

docs/guides/CLI.md

@@ -163,8 +163,15 @@ Take a node out of rotation or empty it entirely (admin):
 containr nodes cordon <id>    # no new pins; running replicas stay
 containr nodes uncordon <id>
 containr nodes drain <id>     # cordon + remove its containers + unpin services
+containr nodes upgrade <id>   # agent pulls its replacement from the server, re-execs
 ```
 
+`nodes upgrade` swaps the agent binary for the one bundled in the running
+server image — the agent downloads it over the API, verifies the sha256 the
+server computed, sanity-checks it, atomically replaces its own executable,
+and re-execs. The node must be online and the server must ship a binary for
+its architecture (`linux-amd64`/`linux-arm64`).
+
 ## Service domains and access
 
 A service can carry multiple hostnames; the default is mirrored to