Define branch base for ticket

[Neokil]

Summary

• add an optional base branch field to the add-ticket flow and run-ticket API
• persist ticket-specific base branches for worktree creation, run context, and GitHub blob links
• add frontend and backend coverage for request wiring, context propagation, and git worktree behavior

Testing

• go test ./...
• golangci-lint run
• cd web && npm test
• cd web && npm run lint

Closes #20