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/update.md

@@ -0,0 +1,22 @@
+---
+description: Update an issue's status, priority, or other fields
+argument-hint: [issue-id] [status]
+---
+
+Update a beads issue.
+
+If arguments are provided:
+- $1: Issue ID
+- $2: New status (open, in_progress, blocked, closed)
+
+If arguments are missing, ask the user for:
+1. Issue ID
+2. What to update (status, priority, assignee, title, description)
+3. New value
+
+Use the beads MCP `update` tool to apply the changes. Show the updated issue to confirm the change.
+
+Common workflows:
+- Start work: Update status to `in_progress`
+- Mark blocked: Update status to `blocked`
+- Reprioritize: Update priority (0-4)