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/close.md

@@ -0,0 +1,18 @@
+---
+description: Close a completed issue
+argument-hint: [issue-id] [reason]
+---
+
+Close a beads issue that's been completed.
+
+If arguments are provided:
+- $1: Issue ID
+- $2+: Completion reason (optional)
+
+If the issue ID is missing, ask for it. Optionally ask for a reason describing what was done.
+
+Use the beads MCP `close` tool to close the issue. Show confirmation with the issue details.
+
+After closing, suggest checking for:
+- Dependent issues that might now be unblocked (use `ready` tool)
+- New work discovered during this task (use `create` tool with `discovered-from` link)