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/create.md

@@ -0,0 +1,19 @@
+---
+description: Create a new issue interactively
+argument-hint: [title] [type] [priority]
+---
+
+Create a new beads issue. If arguments are provided:
+- $1: Issue title
+- $2: Issue type (bug, feature, task, epic, chore)
+- $3: Priority (0-4, where 0=critical, 4=backlog)
+
+If arguments are missing, ask the user for:
+1. Issue title (required)
+2. Issue type (default: task)
+3. Priority (default: 2)
+4. Description (optional)
+
+Use the beads MCP `create` tool to create the issue. Show the created issue ID and details to the user.
+
+Optionally ask if this issue should be linked to another issue (discovered-from, blocks, parent-child, related).