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/ready.md

@@ -0,0 +1,15 @@
+---
+description: Find ready-to-work tasks with no blockers
+---
+
+Use the beads MCP server to find tasks that are ready to work on (no blocking dependencies).
+
+Call the `ready` tool to get a list of unblocked issues. Then present them to the user in a clear format showing:
+- Issue ID
+- Title
+- Priority
+- Issue type
+
+If there are ready tasks, ask the user which one they'd like to work on. If they choose one, use the `update` tool to set its status to `in_progress`.
+
+If there are no ready tasks, suggest checking `blocked` issues or creating a new issue with the `create` tool.