Use gopsutil for cross-platform process detection

Replace pgrep with gopsutil/v4 for pure Go process detection.
Now fully cross-platform (Linux, FreeBSD, macOS, Windows) with
zero external exec calls. Add Taskfile and test fixture.

test/fixture.json

@@ -0,0 +1,36 @@
+{
+  "hook_event_name": "Status",
+  "session_id": "abc123def456",
+  "transcript_path": "/tmp/transcript.json",
+  "cwd": "/root/projects/statusline",
+  "model": {
+    "id": "claude-opus-4-5-20251101",
+    "display_name": "Opus 4.5"
+  },
+  "workspace": {
+    "current_dir": "/root/projects/statusline",
+    "project_dir": "/root/projects/statusline"
+  },
+  "version": "1.0.80",
+  "output_style": {
+    "name": "default"
+  },
+  "cost": {
+    "total_cost_usd": 0.01234,
+    "total_duration_ms": 45000,
+    "total_api_duration_ms": 2300,
+    "total_lines_added": 156,
+    "total_lines_removed": 23
+  },
+  "context_window": {
+    "total_input_tokens": 15234,
+    "total_output_tokens": 4521,
+    "context_window_size": 200000,
+    "current_usage": {
+      "input_tokens": 8500,
+      "output_tokens": 1200,
+      "cache_creation_input_tokens": 5000,
+      "cache_read_input_tokens": 2000
+    }
+  }
+}