fix: Remove unsupported env vars and fix slash commands directory structure

Fixes #42 and #44

- Remove BEADS_ACTOR and BEADS_WORKING_DIR from plugin.json env
  (Claude Code only supports ${CLAUDE_PLUGIN_ROOT} expansion)
- Move commands from .claude-plugin/commands/ to commands/ at root
- Rename command files to remove redundant bd- prefix

This fixes MCP server crashes with literal ${PWD} directory errors
and makes slash commands discoverable in Claude Code.

Amp-Thread-ID: https://ampcode.com/threads/T-69426d6c-25ea-4333-8955-5b624001c8c6
Co-authored-by: Amp <amp@ampcode.com>

commands/stats.md

@@ -0,0 +1,17 @@
+---
+description: Show project statistics and progress
+---
+
+Display statistics about the current beads project.
+
+Use the beads MCP `stats` tool to retrieve project metrics and present them clearly:
+- Total issues by status (open, in_progress, blocked, closed)
+- Issues by priority level
+- Issues by type (bug, feature, task, epic, chore)
+- Completion rate
+- Recently updated issues
+
+Optionally suggest actions based on the stats:
+- High number of blocked issues? Run `/bd-blocked` to investigate
+- No in-progress work? Run `/bd-ready` to find tasks
+- Many open issues? Consider prioritizing with `/bd-update`