feat: add /gsd:stats command for project statistics

Add a new `/gsd:stats` command that displays comprehensive project
statistics including phase progress, plan execution metrics, requirements
completion, git history, and timeline information.

- New command definition: commands/gsd/stats.md
- New workflow: get-shit-done/workflows/stats.md
- CLI handler: `gsd-tools stats [json|table]`
- Stats function in commands.cjs with JSON and table output formats
- 5 new tests covering empty project, phase/plan counting, requirements
  counting, last activity, and table format rendering

Co-authored-by: ashanuoc <ashanuoc@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

commands/gsd/stats.md

@@ -0,0 +1,18 @@
+---
+name: gsd:stats
+description: Display project statistics — phases, plans, requirements, git metrics, and timeline
+allowed-tools:
+  - Read
+  - Bash
+---
+<objective>
+Display comprehensive project statistics including phase progress, plan execution metrics, requirements completion, git history stats, and project timeline.
+</objective>
+
+<execution_context>
+@~/.claude/get-shit-done/workflows/stats.md
+</execution_context>
+
+<process>
+Execute the stats workflow from @~/.claude/get-shit-done/workflows/stats.md end-to-end.
+</process>