fix: Remove unsupported engines field and add runtime version checking

Fixes Claude Code marketplace plugin installation failure (bd-183).

Problem: The plugin.json manifest included an engines field (borrowed from npm)
to specify minimum bd CLI version requirements. However, Claude Code's plugin
manifest schema doesn't recognize this field, causing validation errors when
installing via /plugin marketplace add.

Solution:
1. Remove the engines field from plugin.json
2. Add runtime version checking in the MCP server startup
3. Update documentation to reflect automatic version checking

Changes:
- .claude-plugin/plugin.json: Remove unsupported engines field
- integrations/beads-mcp/src/beads_mcp/bd_client.py:
  - Add BdVersionError exception class
  - Add _check_version() method to validate bd CLI >= 0.9.0
  - Use bd version command (not bd --version)
- integrations/beads-mcp/src/beads_mcp/tools.py:
  - Make _get_client() async to support version checking
  - Update all tool functions to await _get_client()
  - Add version check on first MCP server use
- .claude-plugin/commands/bd-version.md: Update to mention automatic checking
- PLUGIN.md: Document automatic version validation at startup

Benefits:
- Plugin installs successfully via Claude Code marketplace
- Clear error messages if bd CLI version is too old
- Version check happens once per MCP server lifetime (not per command)
- Users get actionable update instructions in error messages

Closes bd-183

.claude-plugin/plugin.json

@@ -16,9 +16,6 @@
     "agent-memory",
     "mcp-server"
   ],
-  "engines": {
-    "beads": ">=0.9.0"
-  },
   "mcpServers": {
     "beads": {
       "command": "uv",