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>

.claude-plugin/plugin.json

@@ -25,10 +25,7 @@
         "run",
         "beads-mcp"
       ],
-      "env": {
-        "BEADS_ACTOR": "${USER}",
-        "BEADS_WORKING_DIR": "${PWD}"
-      }
+      "env": {}
     }
   }
 }