feat(mcp): improve error messages for missing bd CLI

Add helpful installation instructions when bd CLI is not found,
making it clear that the CLI must be installed separately.

Changes:
- Add BdNotFoundError.installation_message() with clear install steps
- Update all BdNotFoundError raises to use new formatted message
- Improve config error message with installation instructions first
- Update tests to match new error message format

Error message now shows:
- Clear explanation that bd CLI is required
- Installation command with curl one-liner
- Link to GitHub installation docs
- Reminder to restart Claude Code after installation

Test results: 90/91 tests passing (1 unrelated path assertion)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

integrations/beads-mcp/uv.lock

@@ -48,7 +48,7 @@ wheels = [
 
 [[package]]
 name = "beads-mcp"
-version = "0.9.3"
+version = "0.9.4"
 source = { editable = "." }
 dependencies = [
     { name = "fastmcp" },