feat: add Prometheus metrics endpoint and Docker image (PLAN.md 4.2)

Add internal/metrics package with dedicated Prometheus registry exposing
SSH connection, auth attempt, session, and build info metrics. Wire into
SSH server (4 instrumentation points) and web server (/metrics endpoint).
Add dockerImage output to flake.nix via dockerTools.buildLayeredImage.
Bump version to 0.7.0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

oubliette.toml.example

@@ -37,6 +37,7 @@ retention_interval = "1h"
 # [web]
 # enabled = true
 # listen_addr = ":8080"
+# metrics_enabled = true
 
 [shell]
 hostname = "ubuntu-server"