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/init.md

@@ -0,0 +1,18 @@
+---
+description: Initialize beads in the current project
+argument-hint: [prefix]
+---
+
+Initialize beads issue tracking in the current directory.
+
+If a prefix is provided as $1, use it as the issue prefix (e.g., "myproject" creates issues like myproject-1, myproject-2). If not provided, the default is the current directory name.
+
+Use the beads MCP `init` tool with the prefix parameter (if provided) to set up a new beads database.
+
+After initialization:
+1. Show the database location
+2. Show the issue prefix that will be used
+3. Explain the basic workflow (or suggest running `/bd-workflow`)
+4. Suggest creating the first issue with `/bd-create`
+
+If beads is already initialized, inform the user and show project stats using the `stats` tool.