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>

PLAN.md

@@ -189,11 +189,12 @@ Goal: Make the web UI great and add operational niceties.
 - Session detail view with full command log
 - Filtering and search
 
-### 4.2 Operational
-- Prometheus metrics endpoint
-- Structured logging (slog)
-- Graceful shutdown
-- Systemd unit file / deployment docs
+### 4.2 Operational ✅
+- Prometheus metrics endpoint ✅
+- Structured logging (slog) ✅
+- Graceful shutdown ✅
+- Docker image (nix dockerTools) ✅
+- Systemd unit file / deployment docs ✅
 
 ### 4.3 GeoIP
 - Embed a lightweight GeoIP database or use an API