Change IssueType and IssueStatus from Literal to str to support
custom types configured via:
bd config set types.custom "agent,molecule,event"
bd config set status.custom "awaiting_review,awaiting_testing"
The CLI handles validation of these values against the configured
options. The MCP layer is just a transport and shouldn't re-validate
what the CLI already validates.
This fixes Pydantic validation errors when listing issues that have
custom types like 'event', 'molecule', or 'agent'.
Built-in types: bug, feature, task, epic, chore
Built-in statuses: open, in_progress, blocked, deferred, closed
68da7c9f78