Add 'gt dashboard' CLI command (hq-s1bg) (#65)

* Add LastActivity calculation for convoy dashboard (hq-x2xy)

Adds internal/activity package with color-coded activity tracking:
- Green: <2 minutes (active)
- Yellow: 2-5 minutes (stale)
- Red: >5 minutes (stuck)

Features:
- Calculate() function returns Info with formatted age and color class
- Helper methods: IsActive(), IsStale(), IsStuck()
- Handles edge cases: zero time, future time (clock skew)

Tests: 8 test functions with 25 sub-tests covering all thresholds.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add convoy dashboard HTML template with Last Activity (hq-fq1g)

Adds internal/web package with convoy dashboard template:
- convoy.html with Last Activity column and color coding
- Green (<2min), Yellow (2-5min), Red (>5min) activity indicators
- htmx auto-refresh every 30 seconds
- Progress bars for convoy completion
- Status indicators for open/closed convoys
- Empty state when no convoys

Also includes internal/activity package (dependency from hq-x2xy):
- Calculate() returns Info with formatted age and color class
- Helper methods: IsActive(), IsStale(), IsStuck()

Tests: 6 template tests + 8 activity tests, all passing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add convoy list handler with activity data (hq-3edt)

Adds HTTP handler that wires convoy dashboard template to real data:
- ConvoyHandler: HTTP handler for GET / rendering convoy dashboard
- LiveConvoyFetcher: Fetches convoys from beads with activity data
- ConvoyFetcher interface: Enables mocking for tests

Features:
- Fetches open convoys from town beads
- Calculates progress (completed/total) from tracked issues
- Gets Last Activity from worker agent beads
- Color codes activity: Green (<2min), Yellow (2-5min), Red (>5min)

Includes dependencies (not yet merged):
- internal/activity: Activity calculation (hq-x2xy)
- internal/web/templates: HTML template (hq-fq1g)

Tests: 5 handler tests + 6 template tests + 8 activity tests = 19 total

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add 'gt dashboard' CLI command (hq-s1bg)

Add dashboard command to start the convoy tracking web server.

Usage: gt dashboard [--port=8080] [--open]

Features:
- --port: Configurable HTTP port (default 8080)
- --open: Auto-open browser on start
- Cross-platform browser launch (darwin/linux/windows)
- Graceful workspace detection

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/dashboard.go

@@ -0,0 +1,91 @@
+package cmd
+
+import (
+	"fmt"
+	"net/http"
+	"os/exec"
+	"runtime"
+
+	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/web"
+	"github.com/steveyegge/gastown/internal/workspace"
+)
+
+var (
+	dashboardPort int
+	dashboardOpen bool
+)
+
+var dashboardCmd = &cobra.Command{
+	Use:     "dashboard",
+	GroupID: GroupDiag,
+	Short:   "Start the convoy tracking web dashboard",
+	Long: `Start a web server that displays the convoy tracking dashboard.
+
+The dashboard shows real-time convoy status with:
+- Convoy list with status indicators
+- Progress tracking for each convoy
+- Last activity indicator (green/yellow/red)
+- Auto-refresh every 30 seconds via htmx
+
+Example:
+  gt dashboard              # Start on default port 8080
+  gt dashboard --port 3000  # Start on port 3000
+  gt dashboard --open       # Start and open browser`,
+	RunE: runDashboard,
+}
+
+func init() {
+	dashboardCmd.Flags().IntVar(&dashboardPort, "port", 8080, "HTTP port to listen on")
+	dashboardCmd.Flags().BoolVar(&dashboardOpen, "open", false, "Open browser automatically")
+	rootCmd.AddCommand(dashboardCmd)
+}
+
+func runDashboard(cmd *cobra.Command, args []string) error {
+	// Verify we're in a workspace
+	if _, err := workspace.FindFromCwdOrError(); err != nil {
+		return fmt.Errorf("not in a Gas Town workspace: %w", err)
+	}
+
+	// Create the live convoy fetcher
+	fetcher, err := web.NewLiveConvoyFetcher()
+	if err != nil {
+		return fmt.Errorf("creating convoy fetcher: %w", err)
+	}
+
+	// Create the handler
+	handler, err := web.NewConvoyHandler(fetcher)
+	if err != nil {
+		return fmt.Errorf("creating convoy handler: %w", err)
+	}
+
+	// Build the URL
+	url := fmt.Sprintf("http://localhost:%d", dashboardPort)
+
+	// Open browser if requested
+	if dashboardOpen {
+		go openBrowser(url)
+	}
+
+	// Start the server
+	fmt.Printf("🚚 Gas Town Dashboard starting at %s\n", url)
+	fmt.Printf("   Press Ctrl+C to stop\n")
+
+	return http.ListenAndServe(fmt.Sprintf(":%d", dashboardPort), handler)
+}
+
+// openBrowser opens the specified URL in the default browser.
+func openBrowser(url string) {
+	var cmd *exec.Cmd
+	switch runtime.GOOS {
+	case "darwin":
+		cmd = exec.Command("open", url)
+	case "linux":
+		cmd = exec.Command("xdg-open", url)
+	case "windows":
+		cmd = exec.Command("cmd", "/c", "start", url)
+	default:
+		return
+	}
+	_ = cmd.Start()
+}